Orks is da best boyz of deze gitz, dem runty boyz get stomped good by tha orkz.
But seriously, Tau have nice models, and are fun to play, if you like a sort of mechanised force give them a shot, I personally think they have some of the nicest looking tanks and variety in the game - but they easily have the worst back story of the lot, seriously.
If you want just a 'cool' army, to collect and paint, I recommend something like Tyranids, nice back story, cool models, and fun to play. Of course if your a real hobbyist imperial guard is a good one, lots of tank models, INSANE variety of forces to choose, and they get a lot of bang for buck.
One race to avoid is necrons, I've heard from necron players that they can get really tiring to play, very little variety apparently.