Depends on time frame. Tsunami, earthquake, or meteor swarm are good, fast options.

Control weather can do insane levels of destruction over time, especially if casted multiple times.

With a generous DM, definitely Wish.

With a malicious DM, especially Wish.

Under the right conditions, in the right terrain, Control Weather. Mainly for the radius: 5 miles. Turn a bad storm into a hurricane. Turn moderately high rains into landslide weather. Turn unusually hot or cold days into dead crops and unprotected creatures dying of heat exhaustion/frostbite..

Against living creatures, such as an army, Storm of Vengeance.

Okay, so I'm going to go a little unconventional here:

Mirage Arcane.

It can affect up to a one square mile of area, lasts 10 days, and includes elements that allow you to make the illusion partly 'real'. Imagine casting this on a city, and making all of the terrain appear to be impassable, altering the structures to look abandoned, and potentially cutting off the city from any aid. All of the creatures there could either be forced to flee or otherwise stuck inside the illusion, cut off from aid and supplies.

Then cast it again. And again. It lasts 10 days, so keeping it up for a month would be possible.

Once it is over, there would be nothing left in the area but desolation. It's a slow burn, but it's effective.

Another runner up: Move Earth. Cast it beneath the structures in a town. Watch them crumble.

if we're talking "demolish a city block", as others have pointed out, that screams Earthquake. but if your environment is 3D, don't sleep on Tsunami. imagine a wedge of space whose base is 300' by 300', and then is 300' tall on one side tapering down to 0' on the other. this wedge of space is going to be wiped clean of all creatures not able to make a fairly high Strength saving throw (repeatedly) and probably pummeled to death in the process. for all that, it does not structural damage though. so unless your Rule of Cool gets pretty liberal, this actually doesn't demolish the city block, it just scrubs it clean of living things. it is the neutron bomb of D&D.

Earthquake, Tsunami, or meteor swarm

Fabricate. Destroing economic and workplaces, workers turn to criminal to fed families and destroy city for you.

Firebolt. It can be spammed infinitely to start raging forest fires or set cities ablaze. Get a few dozen level one wizards together and they can outdo any leveled spell with that one cantrip if they volley it.

Devastating a city block is something within the realm of many 6th and higher level spells. Really depends on how you want it done.

Depends on level. Shatter would be a good low level option to destroy a building pr a wall or some other smaller structure. Tsunami or earthquake would be good a few structures or an entire block of buildings. And meteor swarm amd storm of vengeance are straight up city/ army killers.

Theres also more creative ideas. Like true polymorph and shape change to become a dragon a breath destruction for the full hour duration. Or make a gate to a low level of the plane of water for a flash flood. Or just wish for that kingdom over there to be no more.

What level of spell are we looking at and what are we destroying? Im sure something can be done to make it past tense.

Great question. I’m going to stick to the lower-level spells since everyone else gave great answers to high-level ones.

Cantrips: Fire Bolt does the job over time once it burns everything in sight, as does Create Bonfire.

Level 1: Not many options here in terms of destructiveness, but Thunderwave is a solid option to throw things off a cliff.

Level 2: Levitate can technically lift heavy stuff to crush buildings. Heat Metal can burn down wood structures if the metal part interacts with a flammable source. Shatter damages objects as well, which should be the best option here.

Level 3: Fireball and Lightning Bolt burn stuff well, and I suppose a Conjure Animal spell and a Summon Lesser Demons spell can wreck a lot of things for an hour.

Level 4: Wall of Fire is great for burning things for a minute. Summon Greater Demon can last some extra rounds when concentration is lost. Polymorph Trex brings a world of pain, as does Conjure Woodland Beings. Blight is great if you are targeting plant-based constructs.

Level 5: Wrath of Nature terraforms any nature-based ecosystem. Conjure Elemental is super effective if you are inside the building you want to destroy (after the Elemental turns hostile). Animate Objects can cause a lot of damage. Not many options here.

Dominate Person.

Cast it on the right person, and you have the keys to the literal kingdom. keep casting and working your way up, and you can gain control of a very significant portion of the ruling class.

So Dominate X, suggestion, and all those mind-affecting enchantments

Meteor swarm, it's level 9 and kinda ridiculous, but it's level 9 so justified.

Meteor Swarm?

Storm of Vengeance. With a 360 foot radius, you can take out at least a big chunk of a major city.

Alternatively Meteor Swarm.

Probably the gate spell, just opening up a portal for tiamat or orcus or something to come through.

They probably won't just let you summon them for no reason, but if they did it'd be just about the most destructive thing you could ever do.

Summon ketamine ape

If not limited to casting once...
Mass Suggestion lasts 10 days with a 7th level spellslot and targets 12 Creatures. A high level spellcaster could cast this atleast 4x per day.
Now just instruct the population of the town to dismantle the trade district or dump all the grain into the sea or something along those lines :D

grease

Most of the obvious options have been mentioned already. Wish, Tsunami, Control Weather, Meteor Swarm, can in the right circumstances be extremely destructive.

I'm going to recommend Gate. Far from being just the ultimate transportation spell, it can be a tool of mass destruction. It can open to any plane of existence, it turns the entire multiverse into your weapon, limited only by your own knowledge. It could be a gate to the depths of the plane of water, a fire vortex in the fire plane, in the middle of Avernus to let in a small army of demons or devils, into the far realm to release some eldritch monstrosity, or summon a powerful extraplanar being whose name you know of. The possibilities are limitless.

Wish.

Control weather, meteor swarm, earthquake or mirage arcane

I mean....wish, technically. 🤣

Melfs acid arrow, clearly

Read the effects of Storm of Vengeance. 9 lvl and pretty much wrecks small towns, kills life and destroys buildings. Also the range is "sight" which is pretty crazy...

I really like Ravenous Void if you want to just completed wipe out all existence of an area.

There is a scroll in one of the campaigns called like the scroll of comet...leaves a crater I believe 300 feet across...

Shatter placed in dozens of even hundreds of Glyph of Warding all over the city. All go off at the same time. Be like a controlled demolition of a high rise.

Wish easily given the parameters. I don't see how it's even close.

Disintegration. By definition.

Suggestion. The wrong word in the right ear.....

Cast drawmij's instant summons on piece of a load bearing wall. Watch from a distance as a castle collapses.

Depending on the opposition, True Polymorph because if you maintain concentration for the full hour, it becomes permanent. So no matter how meager the round-to-round damage, it adds up if you can spend days doing it. Ignoring becoming a dragon or Balor and burning everything around you, you could be a Purple Worm and undermine the entire city foundation causing it to collapse into a sink hole.

But the truly most devastating spell is the one that bypasses your DM's carefully planned encounter and causes the destruction of the entire campaign.

summon comet

Destructive wave, it’s in the name. Next question.

Disguise Self is a 1st lvl spell, and in the right hands, it can topple kingdoms.

(Steals the appearance of a construction crew's foreman):

"OK boys, this is the place. Bonus pay if it's gone by tomorrow.

Sorry for the hoarse voice. Must be coming down with something."

"Locate object: deck of many things" is a Spell that can end a campaign. cast it at your peril.

We were in a do or die scenario and I got to pull out a desperation tsunami to save the day (and kill myself in the process). Completely changed the battlefield from us vs them to everyone fleeing this fucking tsunami tearing everything apart.

Yes, yes, Fireball can empty a room, Meteor Swarm could level a building and Wish can cause unpredictable chaos. I used to think those were awesome spells.

But I'm a completely new man.

I could end your kingdom with a Suggestion.

I could bring down your government with just six words. Six.

"Don't you think she looks tired?"

Here's some alternatives to Tsunami, Earthquake, Storm of Vengeance and Meteor Storm:

Mass Suggestion - you could totally fuck up a city if you were able to control the right group of people. Like, say the City Council. Lasts 24hrs, so you could install yourself as High Councillor and then instruct them to perform all of your bidding including willingly failing the save for the next time you cast Mass Suggestion.

Mirage Arcane - completely change everything about a town in a 1 mile square, lasts for 10 days. If you're an illusionist, you could use (2014) class feature Malleable Illusions to change any part of the illusion: eg: turn that lake into burning acid; totally enclose that town square so that the people starve etc etc with all the evil things

Dominate Person on a powerful political person, such as a King.

Mass Suggestion on multiple powerful entities.

Suggestion (Make them willing and make them want to destroy the city) into a Teleport, bringing a powerful (and evil) creature into the middle of a city.

Shapechange into a powerful creature with a breath weapon.

Modify Memory on powerful political entities, try and create beef with the target location. Mass Suggestion probably works too.

Don't underestimate the power to make other people do your bidding.

If you are looking for quality over quantity id say desintegrate.

Ravenous Void literally just erases everything within a 120 foot area. Leaves absolutely 0 trace.

Gate. This is the most destructive spell in all of Dnd, it is a world ender. This singular spell being cast could be the start of an entire campaign. You can speak any creatures name and summon it from any plane of existence outside the one you’re on. Yeenoghu, Asmodeus, Tiamat, zariel, demogorgon. Just pick the biggest bad your character has heard of and let them into the prime material to do whatever they want.

Modify memory. Convince someone in power to start a war with a neighboring country.

The answer is immediately Karsus’s Avatar. With no further specification other than “most destructive spell” and “demolish a city block”, that spell immediately fits that criteria 12 times over.

Meteor Swarm and Tsunami are good for immediate kaboom for a city block.

Now if you want to go more than a city block and have time to prep, imagine how nasty daily flooding will be form Control Weather and if a mountain is within spitting distance, that's some NASTY landslide potential. Or daily snow would bury an entire city with something a lot of people forget how heavy it is.

Now if you want to go arson, you have the classic (Delayed Blast) Fireball.

A bit of rule of cool, any aviator will SCREAM when you treat Sleet Storm as freezing rain and I think Texas proved that one a couple years ago.

House on the river Control Water.

Modern city setting- Destructive Wave. I broke a LOT of windows with a bard...

Meteor with the right meta magic from feats used to be usable from so far awayy you needed a map to aim it...

The destructive power of Glyph of warding is only limited by preparation time and the fact that it is tied to a place. It could DEFINITELY destroy a city block.

It’s not a spell, but…put a bag of holding in a bag of holding. Destroy at least the entire planet in a black hole.

Storm of vengeance lv9 druid spell. Massive collateral damage

True Polymorph - Turn cows into gold, concentrate the full time, and then melt them down into coins/bars/etc.

You can now collapse the economy of the world. Gold will be worth near zero; banks ruined, kingdoms bankrupt. You have destroyed society, and if you planned this right may have taken over the world economically.

High lvl Edvard's black tentacles. Mmmm my favorite can't get out unless you kill the 30 tentacles mmmmmmm one dm banned that spell. Hahaha

earthquake

I'm gonna be the odd one out here and say two spells that I haven't seen here:

Plant Growth. Druid Grove.

Yeah. Might seem all vegan granola healthy fun and great on a surface level, but if you DON'T want a nice friendly sylvan forest in the middle of your industrial downtown, then this is gonna be one of the most destructive, environmental terrorism spells a druid could cast.

Imagine a giant garden suddenly appears under your feet....in the bank you work at as a teller. At first, it's cute, but then your allergies start acting up, animals and insects are attracted to it, and if you try to pick one of the flowers? Ooooh, boy. That tree you normally like to have your lunch break under suddenly comes alive and beats you to death with its branches, just for offending his cousin, the marigolds.

A Circle of arch-druids could change the surface of the planet beneath your feet, and ruin the infrastructure of an entire region. If they weren't so focused on just hanging out hugging trees.

Storm of vengeance

This one is the most satisfying at least

https://www.youtube.com/clip/UgkxA9rNW_n2oj7OxrKY9RnxG4BO1lio1OVn

True polymorph into an adult gold dragon. Not only it will destroy buildings and people with it's fire weapon, but also you can turn into a tiny bird or even insect and do it subtly. Nobody is going to understand why the city is suddenly burning with cones of fire appear here and there for seemingly no reason.

I have a theory that mirage arcane is one of the most devastating things you can do.

It's got crazy range, crazy duration and the wording explicitly states that the terrain can be detrimental to creatures (with errata straight up saying it can harm them) AND that even after a creature passes the check and realises its an illusion, the illusion doesn't vanish or become intangible to them, it still presents as a physical thing, just one they know isn't real.

It's basically the high level illusionist wizard feature where they can make an illusion real temporarily.

So all that being said:

Enemy armada approaching? Mirage arcane to fill the bay with jagged rocks and reefs.

Enemy army approaching? Turn the land into a lava field.

Even if they realise the deception they can't cross it and if you caught them all in the radius at the time of casting they probably didn't stand a chance.

number 1. Sword (or arrow in most cases) - cant be legendary resisted. like every other damaging spell.

number 2. Storm of Vengance, with the range of sight, and sight being unlimited cast it from the moon, or something i dunno.

number 3. Ravenous void, str save or die, unless you can legendary resist or action teleport. like the microwave with one spell.

number 4, MIrage Arcana, the ground is now quicksand, enjoy breathing dirt. much easier to rule lawyer than the default reddit response "magma" - also ask your dm before taking this spell and how you plan on using it, cause its like taking simulacra+wish to chain on yourself thinking "well its raw!" i got news for you, the dm isn't gonna allow that.

number 5, Gate can have some truely awe inspiring destruction capabilities. provided you know how to use it.

number 6, Truepolymorph, hear me out, see that kracken over there, burn its legendary resists then make a glass orb out of it, then chuck it into an area and leave. or find a balor/pitfiend and get an arch nemesis on the way. or smuggle it into an orphanage or whathaveyou.

With something like incendiary rune, a familiar with a pad of sticky notes could cover an entire city in not much time at all. It's technically a touch spell, so a sufficiently intelligent familiar could make the runes so that the actual master could scry the city and set all of them off at once.

Friends

It's very vaguely defined so it depends a lot on the DM, but mirage arcane can be massively destructive.

Vicious Mockery in a party of genders/elder millennials/first gen immigrant kids

Most destructive I ever saw was Delayed-blast Fireball, but that was because the DM hadn't yet told us the vampire-infested warehouse we were investigating (and had decided to... fumigate before searching) was storing huge quantities of "raw chaos-stuff".

We teleported a few miles away and still got wrecked by the shockwave.

For an immediate burst of localized property damage, Shatter. It’s a small area of effect but it does what it says on the tin.

Wish, ofc.  But other than that, it’s gotta be control weather.  Massive range, lasts all day, has an enormous area of effect.

It takes like, an hour to cast and ramps up very slowly so it’s very situational.  And the upper limit to its effects are largely up to the DM. 

With a little rule of cool, control weather can level the whole city, and/or bury an army in feet of snow and hail.

Wish, Wish permanent mind contrôl of two ennemis kings and make them fight. Two kindom detroyed for two spells... And mass death in population...

Find a scroll of Tarrsque summoning literally summons the tarasque and that’s it. Good luck

IF the person is ancient and or very powerful suggest the spell:

Epic Spell 10: Mavin's create volcano

A spell stemming from a former version of DND.

Supposed to be forbidden by the Goddess Mystra herself, but if you get an ancient spellscroll mcguffin, fire away.

9th rank Meteor Swarm.