I must confess, I have only played the DoW games, so understand that my experience is that alone. Also, I may not be 1337 for this conversation, my miniature gaming experience is sorely lacking. So my opinions are pieced together from some wikipedia articles and the video games. If I am incorrect in details, my apologies.
But I gotta side with the Tau. Utilitarianism is the best moral philosophy in the world, and the Tau try for it. Utilitarianism is the only philosophy I will go to the mat defending. That, in the Tau, I gotta respect. Every other faction I generally find humorous because they are so ridiculous, but the Tau are someone I can relate to. And their saints compared to everyone else...well, the Eldar seem Decent-If-Haughty to me, but the Tau are still the heroes to root for.
The Tau may be a tiny faction, but that just improves there appeal for me: There the scrappy underdogs! There victories are so much sweeter because they are at a disadvantage. Oh, and they improved in technology so rapidly, and took a bunch of other races under there wing, like the Vespids and the oh so awesome for a evolutionary science geek Kroot. I think there more analogous to humans in other Science Fiction then the human factions are.
Lastly, I am kinda prone in RTS to answering all threats with more ranged stuff, so the Tau are a sheer joy. And my favorite end game is huge amounts of power generators, and never ending drone assault, plus I love just watching the krootox and gnarloc takes obscene beatings and not dying. I think the sides are all balanced, so I gotta say I gravitate to there play style
Second may be Sisters of Battle, because I love there artillery, the sheer amount of fire burning everything in sight actually makes me giggle maniacally, and they have some of the funniest and coolest lines in the game ("Faster then Blasphemy", anything with the word, "Echlesiarchy", etc).