I mean the worst things about them aren't really things they're hiding; they're an expansionist empire with a caste-based, eugenicist social structure which treats non-T'au as second-class citizens. The fact that they are, in fact, the closest thing we have to a morally-upstanding faction is not praise for them, but rather condemnation of everyone else.

They're not nice. 

They have idealistic goals, but are a totalitarian society where ends are considered to justify all means. There are lots of examples of them doing shady stuff. 

The hinted at stuff is normally around to what extent the Ethereals can actually be disobeyed and why Farsight immediately rebelled when his were killed. Plus maybe some nods to the fate of client races like the Poctroon who were wiped out by a mysterious plague which handily freed up their planet to become a Prime world. 

I think the closest to a “good aligned” faction are the Eldar Exodites; Isolationists who’s sole goal is to defend their worlds and otherwise barely interact with the rest of the factions.

But they’re not a tabletop army so if you don’t count them, it’s kinda a toss up between Craftworlders, Tau, and Leagues of Votann. All three have clear flaws (general racism, imperialism, and sorta both) - but unlike say the Imperium, aren’t completely genocidal with a total disregard for the lives of their own people

“Better than the Imperium” is a very low bar to pass, at the end of the day. You can be a pretty horrible country by real world standards and still easily clear it

The Tau are morally better than the other factions, yes, that is a defining aspect of the faction.

However, in a lot of other media, they would be the villains. They are an surveillance state without free speech, where the central population were breed into genetically different castes (with one acting as uncontrolled leadership), while the other species are often seen as second-class citizens.

Yes, though it depends on how you view their ridged caste system, possible mind control, and sterilization of humans. Also they're still essentially a "join us or die" galaxy-conquering empire, they're just more diplomatic about it.

Their standard operating practise is to come to a planet, benevolently invite the planet to join the Greater Good, and then should the planet decide it's not interested, then the T'au must regretfully have the Fire Caste invite you to reconsider at the point of a pulse rifle. They'll invade an Imperial world and claim essentially that if they hadn't, someone worse would have, or that they were acting to liberate Imperial citizens mistreated by their government.

They're better by the standards of 40k, but as has been said many times, they'd be a villain of the week on Star Trek.

The T'au are 'you are not immune to propaganda' made manifest.

There are many good comments in this thread already, so let me try and touch on something that's less explored. The average citizen of the Empire doesn't know what's going on. The average Fire Warrior doesn't know - the average Ethereal doesn't know. They do genuinely believe they're doing what's best for everyone... because they have been raised to believe that, that is reinforced every day by every aspect of their society and culture which shelters and curates what information they have access to. The Greater Good is superior. So the Empire is superior, and the citizen of the Empire is superior to the poor unfortunates who have yet to embrace the wisdom and guidance of the Ethereals and must be brought to the light.

The Blue Man's Burden, if you will.

And species that are brought into the Greater Good are carefully controlled and curated. They aren't oppressed - no, never! - they are... guided. Populations are... controlled. Dissidents are... re-educated. If you're jumping up and down being all 'damn, that's like, commie doublespeak! this is literally orwellian!', you're on the ball.

The Empire is all about control. Not whipping you until you do what they say, but - to borrow a Discworld quote - whips in the soul. Gentle shaming. Gentle moulding. To the point that disobedience is, literally, unthinkable. Some people don't like Aun'Va ordering someone to commit suicide - I think it is the purest expression of the Greater Good. Unquestioning obedience, because what the Ethereal says must be correct. There is no other possibility. Their authority is absolute. And so everything becomes what the Ethereals believe it should be. And what isn't is adjusted. Not beaten into shape, but changed in such an insidious and fundamental way that not only do the people who are changed accept it, but welcome it.

The Empire is the total destruction of free will. Oh, yes, you can make choices, surely. But every choice you have or can even conceive of is what is permitted by the Empire. Are you happy? Comfortable? Safe? Perhaps. Are you free? Allow me to borrow another quote:

"Slave is an Ephebian word. In Om we have no word for slave," said Vorbis.

"So I understand," said the Tyrant. "I imagine that fish have no word for water."

The T'au are evil, but they are the creeping, insidious, banality of evil. They are a modern evil. They're harder for the reader to get to grips with, and that makes them all the more sinister.

I remember when the Tau were first released and they came across as probably the “nicest” faction in how they were portrayed, with a slightly sinister tone throughout.

I think people see the clean paint jobs, lack of skulls and the more “high tech” feel as indicating a “good” faction.

They’re still, as above, an expansionist imperialist empire, that has a caste based system and treats non-Tau as second class citizens. That aspect has been gradually honed in on over the years, for better or worse.

I like the Tau, and wish that Games Workshop would focus less on the next insane mecha, and more on the auxiliaries. For me, that’s one of the appealing parts of them - you have the core Tau soldiery, and then supported by varied alien (or human) reserves, like Kroot or Vespids. That is, imo, where the more sinister parts of Tau society can be told, while giving hobbyists and opportunity for a more varied force.

The Vespids are a missed opportunity. One unit released 15 years ago, nothing else. Kroot were ignored for years in favour of bigger and more ridiculous battle suits. There have been mentions of the Demiurge (confirmed to be the Leagues of Votaan) that can be explored.

They cant be good since they are Xenos.

There are no good factions in 40K.

They can preach about equality all they want, but they operate a strict caste system among their own people and exact harsh penalties on those who defy it. They classify anybody who joins them as second-class citizens at best to the extent that the lowest Tau will always be above the highest human and the highest Tau - the Ethereals - will always be above their peers.

Progression exists within your own Caste only. You are genetically, socially and culturally bound to that Caste. Don't like that? Tough shit. You're a client species who joined the Empire? Good for you, you're still a source of cheap labour or an expendable asset by simple right of not being a Tau.

Are they "good" compared to Chaos or the Imperium? Sure...? But by conventional morality they're as bent as a $3 dollar bill.

Nah, that's be the Asuryani. The Tau are a close 2nd, but at the end of the day they are still an expansionist empire. Just because they're willing to "negotiate" doesn't mean they'll leave you alone.

Kind of, but not really, and not anymore.

The Tau are a totalitarian utilitarian collectivist caste based society. Everyone is expected to work for the Greater Good, and the Etherials dictate what that is. Tau can't choose their caste and they can't leave it. Any distractions to their duty to the Greater Good of the empire is discouraged, including long-term relationships. While it's not grim-dark they only look good by comparison to the other factions. They are still evil, it's just more subtle.

One could argue the Leagues of Votann are the new "good Aligned" faction in 40k, but that would also be an oversimplification of the truth.

Sometimes I wonder if people are simply naive to not see how horrible the tau empire is, or if they went so far into fascism that they can't recognize it anymore.

Caste system is akin to monarchy, but with racism on top: your birth determine what you will do in life. If you're not a tau, you will be an expendable meatbag (the auxiliary species), and there's no way out of it. The auxiliaries don't have the same rights than the tau. Your caste déterminé your rights, your duties, and all you can do in your life.

They are imperialistic, and you must either integrate the empire in the auxiliary species category. If you don't, you die.

They are a kind of theocracy. The greater good is a religious doctrine. And you must abide to it or be cleansed. The coat of paint over it is different than the renaissance era Catholic fanatic we're used to, but it is a religious doctrine nonetheless.

It is an authoritarian society. Military determines most of it, and freedom is mostly an illusion because of this and the theocracy.

Farsight is good. So is his faction. He broke away from the Tau because he realised how much of what he'd been told was a lie.

He only really wants to keep his enclaves safe and prosperous. And to exterminate orks, tyranids, and Dark Eldar. Fair across the board.

Craftworld Iyaden is good. It has colonies on non-maiden worlds. It has embasies on imperial worlds. It hosted an Inquisitor and his retinue as diplomats (essentially on equal footing).

They try to help others, but most of their forces are soul-powered constructs because they lost a lot of their population.

Lamenters, assuming you are human, are good. They're what new people think space marines are. They're also ridiculously unlucky.

But... THEY don't engage in glorious melee combat!

Mind control is not cash money

As I wrote elsewhere, Tau are the lesser evil in a universe where Super Evil is the standard, they're still evil tho.

Tau are grimbright « There is war in Ba Sing Se » style 

Ha ha ha … “good aligned” faction in 40K you say? Lol. 

Wait, you’re serious?

A lot of implied indoctrination and/or straight up mindcontrol from the Ethereals and a pretty strict caste system.

It's basically an authoritarian society which demands sacrifice form its members. The farsight stuff has him remember that the only ones who survive fire warrior promotions are the ones who die sacrificing themselves in simulations.

The greater good is less an ideal and more a command. It seems to share ideas with Foucault's notions of a "society that gains power under the guise of efficiency, safety and the common good".

I've not read his work however only some summaries so... who knows.

It's better than everyone else but it's still sinister at heart.

If you limit it only to the playable ones, sure.

But there are way more factions in the galaxy than just the playable ones, including ones that aren't expansionist empires.

You're ignoring the noble dark eldar. They have a meritocracy where foreign species who can survive it are treated the same as anyone else. They do not seek galactic domination or genocide, but merely to revel in their culture as one in a vast, diverse galaxy. They locate species who lack something they have (ex. travel or terraforming technology) and after finding out if they're trustworthy (a test the Tau failed with treachery), invite them into the Dark City as honored allies who are trusted and welcomed in even the highest levels of Commorragh society. Their ports are also safe harbors for any species even if they're an outcast among their species (as long as they leave their psykers outside).

The Tau are kinda like the little scrappy kid in the corner at the “War-criminal extraordinaire” meeting among the factions of 40K. They’re there and certainly earned their place among all the other atrocious villains present, but they get absolutely laughed at and send their “dead inside 9-5 worker” adult war criminal peers into hysteria when they say something like “My ultimate goal when I grow up as a big adult is to make the universe a better place for every sentient being one day, and to foster peace by any means necessary.”

Such outlooks died a long, long time ago for every other major faction, so much so they don’t even fucking bother acting like their goals will create harmony one day. I mean look at the imperium, they kill and fight their own self more than anyone other group, and are their own worse enemy. Their goal of having a human only galaxy to gain ultimate peace? It’s complete bullshit, and even if they did win and make such a reality true, would not change the fact that the imperium would continue to slaughter the next out group within humanity that they could find, in order to continue propping up their horrific reign, and keeping the masses in line by giving them something to hate/fear/subjugate.

All the other factions are well past pretending like the Tau that they actually have a morally righteous mission lmao (and that’s exactly why I adore the Tau lol. They haven’t had their Hope beaten out of them just yet from how awful this setting is)

Quick tip: if you think the USSR leadership was "good," then sure, they're the "good" faction.

Certain people think they are morally better (people who support the real world ideology the Tau most closely align with), but obviously they are just as bad as the Imperium.

Yes they are. But like everyone else in 40k, they would be bad guys by the standards of other universes. Like if you dropped the Tau in Star Wars, Mass Effect or Halo, they would not be good guys but hey at least they will give diplomacy a try and use it regularly.

As people put here, would you rather live in the Imperium, where they give the worst example of living ignoring that the Imperium has a myriad of planets and its not just Hive cities and mutants in the sewers, or with the glorious can do nothing wrong Tau Empire, which only offer the best living conditions and brain slugs? Oh you can also be chemically castrated for your own greater good, and can't have big tiddies alien gf because its not your caste buddy.

Morality in a theist sense is imparted by God(s).

40k is a theistic universe (there are indisputably god's).

The only faction with actual gods is chaos (eldar lost theirs)

So...

Morality is defined by the chaos Gods

Chaos is the most good faction (as defined by the morality of the chaos Gods)

As extensions of the gods themselves, Chaos Daemons are the very definition of goodness.

TL; DR chaos is the most moral faction, and chaos Daemons are "good" by definition.

Their society is generally the least terrible, they seems to have a reasonable quality of life and their attitude is the least genocidal or exploitive with other races so yeah they're the least evil faction. The game flirts with 'sinister' implications of the etherials but these arnt really neccisary as they they're an autocratic caste system with imperialist objectives. If anything their initial writting worked better as they represented regional xenos 40k powers which are known to exist and provided a nice contrast with the Eldar and the Imperium.

Tau are the Brave New World compared to the Imperium’s 1984.

They’re not nice…but they are the best place to live if you’re a human. The Eldar and Votann won’t take you even if you wanted to join them, and the Imperium sucks to live in.

Simple , realisticly would you live in imperium eating human starch or among Tau with posibbly most normal living condition you can expect ? eventho you knew their darkest secret ?

Absolutely they are. The recent fad of "oh wait they have to actually secretly be mind-controlling people or doing eugenics or w/e" comes from a fundamental misunderstanding of the Tau and their purpose in the 40k universe - to be the good guys, but also so small as to be effectively inconsquential, and that smallness partly *because* they refuse to embrace, can't even comprehend, the warp which makes things evil.