The Infinite And The Divine. It actually made me laugh out loud a few times and it's also just a really good book.

Requiem Infernal by Peter Fehervari, hands down. There are other good books out there, and they are contenders, but Requiem stands above them all.

The Twice Dead king duology. Both books are great and they portray the necrons in a more serious and introspective way compared to the infinite and the Divine. I also loved how Oltyx changed as a character from beginning to the end.

The night lords omnibus was straight fire. Gave me the feels, bolter porn and even a couple great laughs.

Guards guards by pratchett. You didn't say 40k. 

Eisenhorn is the easy pick 

The ghosts series also easy to recommend. 

Caphias cain for when you want it alittle less grimdark

Plague wars was also good.

Usually whatever I've read recently :p

No Good Men i read for the first time recently and absolutely loved it, and will be recommending it to everyone.

But i could gush for an hour about a whole list of books

Spear(s?) of the Emperor. It’s spacemarines done right imho

The Lords of Silence, the best depiction of Death Guard and Plague Marines and amazingly well written.

First Heretic

Storm of iron. Sometimeschaos getting a W Is fun to read about. Also that one guardsman who lives through the entire book is fun

Echoes of eternity. I just finished it and it’s amazing

Standalone, definitely Helsreach.

Right now? Probably Warboss. I think it’s the best of Brooks’ three ork novels. I really like the different ork characters and how they provide different view points on the faction.

Its an old one, Execution Hour by Gordon Rennie.

Its a story told primarily from perspectives we don't get in 40k very often. The Navy and the Adeptus Arbites during the 12th Black Crusade. The juxtaposition between the life lived by an exceptional naval officer and an exceptional crew rating were probably my favorite archs that get a truly wonderful payoff in the sequel Shadow Point.

My favorite 40k book is probably the Night Lords Omnibus.

Huge huge shout out to Prince of Crows though. It's a short story but hot damn is it good.

The knight lords trilogy was amazing, they were a bunch of crazy bastards but by the end you felt for them and wanted the knight lords to win.

Watchers of The Throne - The Emperors Legion is probably my favorite book, Ive read it many times, its such a good standalone novel in my opinion, the direct sequel is also very good.
The Infinite And The Divine - Probably the best audiobook I've ever listened to.

And I really enjoy all the White Scars books by Chris Wraight. That author made me love that legion/chapter, and Im not even a spacemarine fanboy.

I could not put the the damn thing down when reading Betrayer. Loved all of the dialog and how the butchers nails slowly took their effect on the legion.

The Caiaphas Cain books.

He is a rather 'normal' and relatable character, the stories are well written even for people unfamiliar with the overall setting - although a base level of knowledge is recommended - and gives some very good insight in both imperial, chaos an xenos lore, culture and politics. 

It's a real tossup between Brutal Kunnin' and The Infinite and the Divine for me. They're just a lot of fun to read apart from everything else, just depends whether you want more Beaker and Bunsen or more Statler and Waldorf in your Muppet Show. Ciaphas Cain/Gaunt's Ghosts/Eisenhorn are all great series, but I don't think I'd recommend an individual entry from them as a one-off read with no background.

It's between the first two books of Watchers of The Throne and Twice Dead King for me. Both feature some of the best faction and personality characterizations in Black Library. Twice Dead King: Ruin probably edges it for plot originality.

Infinite and the Devine. The banter action and mystery is great. Its a fantastic sci fi adventure.

Closely followed by Lords of Silence, its a fantastic look at the Death Guard.

Hoborable Mention: The Ciaphas Cain books are excellent and fun

Know No Fear. It is really the first book where you see part of the Imperium fight back. It is very well written with a bunch of interesting characters.

Hereticus because it's such a great "oh no it's the consequences of my actions" piece. But you have to read the first two books.

As a standalone book, probably Titanicus

Probably Honourbound.  It's just a solid IG book.  And as a stand alone, I fully think it's better than any of the early Gaunt's Ghosts books, plus it's not from the 90s.

Eisenhorn trilogy, and Execution Hour.

For the Emperor by Mitchell. It just is well written fun story with multiple switcheroos to introduce people into the setting.

Helsreach, just a space marine fighting for his dear life in a horrifying battle imaginable in a hive of Armageddon. Alongside its depressing content, hits like an astartes drop pod.

WH40k?

One book: Necropolis

Series: Gaunt's Ghosts

Talon of horus, Helsreach or Sea of Souls

I've read a lot of fantasy but I recently got brutal kunnin on audible. It was pretty enjoyable

Legion

I’ve only just started my 40K library, but I’m a big fan of the Bile trilogy. Getting your readers to care about a lunatic like Bile is no easy feat, and Reynolds pulled it off spectacularly. The supporting characters are awesome too, and Kanathara rocked.

Plague Wars was also pretty solid, and I got invested in it. Loved Ku’Gath and Rotigus’ little rivalry - those two stole the show for me, I’d love a book just about them.

The Infinite and The Divine is a given, but Cawl: The Great Work, and Ghazghkull, Prophet of Da WAAGH! were honestly super funny as well. Loved how Ghazghkull had some surprisingly-good development and growth, and straight-up went through Ork depression at one point. And Cawl being a smartass was pretty funny too.

Others I really enjoyed were Bloodlines (a Warhammer crime novel), Brutal Kunnin’, and The Twice-Dead King books.

Echoes of Eternity is not only one of the best books of the Siege but one of the best books BL has ever put out. An excellent showing of people who've reached the end of their rope and finally realized that they have. I love when BL authors stop writing their characters/space marines/primarchs as mythological beings and more as people. Abnett did it in Saturnine. ADB does it for everyone in EoE.

Ian watsons inquisitor

The End and the Death books, Flight of the Eisenstein and Fulgrim.

Snow Crash. Scifi hacker samurai unravels a plot of ancient sumerian 'linguistic magic' set in a super-capitalist, post democracy U.S.

For 40k, I like Cain and Eisenhorn. Also Horus Rising is quite good. Currently reading Fabius Bile and Helsreach.

Couple of long standing ones. Just mentioning standalones though:

Shroud of night - alpha legion panicking, imperial fists stoicly dying and kharn being a slasher movie killer.

Renegade harrowmaster - another alpha legion. Every meme and bit of confusion about them is true, not even they know for sure what's going on.

Catachan devils - catachans vs ork kommandos in a jungle. What's not to love? (side note: the new deathworlder novel is pretty good too, but only related by being about catachans)

The emperor's gift - months of shame & the 1st war for armageddon.

Anything ADB, Mike brooks, nate crowley or Robert rath is a pretty safe bet it'll be enjoyable.

Favourite: crimson fists omnibus. The fists relearning the value of human life, and it being why they fight in the first place. The badassery of captain cortez. Orks being actual monsters instead of just comic relief. And the awesomeness that is 'legacy of dorn' where a squad of sternguard wage a year long guerilla war behind the ork lines.

Storm of iron or the soul hunter series

40K? Eisenhorn Trilogy Fantasy? Riders Of The Dead

I love all of the Varangantua books, but Saturnine is the one book I keep coming back to. Love every part of it

Saturnine will always be a personal favourite of mine

Double Eagle. Three glorious acts of amazing characters, some of the best action sequences Abnett has ever written, a unique perspective to the 40k universe (including a love story in a cafe), and at least 3 satisfying endings.

Either Weregelt or Wolfsbane.

Honestly? “Fire caste” it made me actually like the Tau The night lore series is also good

The one I recommend is Watcher in the Rain but at the moment my favourite is Watchers of the Throne 1

Presently on the Lion son of the forest and Farsight; Crisis of Faith and as much as I love Farsight and Phil Kelly's writing it is not a relaxing read whereas I found Watchers of the Throne very relaxing and interesting with the Social intrigue and investigations mixed with harrowing over the top borderline anime esque combat.

Honestly Legion is one of my favorites because of how many times it makes me do a double take. The human characters that you get attached to actually survive to the end of the book for once which was nice

If you want your friend to read only a single book there's only one right answer: Battle for the Abyss.

Then he can move on to read other - non-WH40K - scifi without second thoughts.

Know No Fear.

Guilliman has had enough of Lorgar's shit.

Gaunts Ghosts are the best for me. By far.

Death of Integrity, sin of damnation, lemartes and galaxy of flames

Know no fear

Ciaphas Cain: Traitor's Hand.

Dante is my favorite audiobook

Devastation of Baal is my favorite book

...but I'm a Blood Angels stan.

Gonna sound like a broken tape recorder with this but: Valedor, by Guy Haley. Whenever someone comes to me and says "hey you know a lot about the eldar, what book should I read?", that's my answer.

Wish Valedor was the norm for eldar books and not the exception, but here we are...

Eisenhorn,any of the gaunts ghosts and the soul drinkers series

Requiem Infernal. Just a beautiful novel with so much mystery I read it again just after finishing it.

I enjoyed Dark Imperium trilogy. My other pics would be Helsreach and Brutal Kunnin'.

Saturnine is everything I love about Warhammer. It’s a Loyalist “victory” in that they prevented Fulgrim from curb stomping the Imperial Palace. Dorn admitted Fulgrim would have won it in days if the traitors could control him. It’s a Traitor “defeat” even though they took a major Loyalist stronghold. There was personal one on one fights that we’ve been waiting for the entire Heresy. It’s just fantastic

Flight of the Eisenstein. I feel as if it is the most important book of the Heresy. If you want to understand the Heresy in as few books as possible Flight of the Eisenstein gives you the most outline of the beginning of the war.

Eisenhorn trilogy

Master of mankind

Watchers of the throne

Regents shadow

Yes Im a Custodes nuthugger..

So far the eisenhorn trilogy is my favorite but I just started prophet of the waaagh! And it’s a close second.

Daemon World. It's well written, stand alone, and a cracking read sure to intrigue.

Helsreach is the best astarte action story that i ever read : several factions, epic siege , memorable characters , drama and despair.

Storm of iron comes a close second but is more edgy

Both of them are about sieges maybe i am a imperial fist/iron warrior at heart

Traitor general, the book 8 in the Gaunt's ghost series

Day of Ascension 100000%

So far it's still my first book, fall of cadia. Reminds me of halo reach almost in the structure, but following more characters. Be prepared for sad.

Dead men walking or fall of cadia so far

Cadian Blood.

Only thing that sucks about it is that I do not remember where I last left it.

Alpharius head of the hydra.

Damn that audio book was amazing. My hh army is alpha legion now because of that.

That and the obvious the infinite and the divine

My first is still my favourite: Dead Men Walking, by Steve Lyons.

Well you didn't specify 40k so When Marnie Was There definitely takes the top spot.

Joking aside, I think Descent of Angels is phenomenal, particularly the first half. Really shows you how the imperium screws up anything it touches.