Just started picking the game up with a group of friends in the last few months. I play Tyranids, largely because I like their playstyle, they remind me of Zerg (my most played race in Starcraft), and I like their back story. Something about an unstoppable, single minded, hive of alien predators from beyond our galaxy, seeking only to devour entire worlds appeals to me more than pretty much any other army except maybe the Tau.
I'd say my least favourite are the space marines of just about every variety. Can't totally put my finger on it, but it's likely tied into the whole stereotypical superhuman badass in power armour. Maybe if I had gotten into the game when it was new the power armour wouldn't put me off as much, but now everyone seems to do power armour if it's even remotely space related. The religious fanaticism doesn't help either. And I really don't like their look either. The vehicles and things generally look pretty cool, but the actual marines themselves leave a lot to be desired.