Like BoB, I've tried most, so I'd have to say Underworld. I thought they'd suck narly as much as the Gobo team, but a guy playing the team in our league has shown me the error in my thinking (the sucky team of the three new ones is Chaos Pact. Played them for 35 games and the more I played, the more they made me want to cry).
There are a few unofficial teams I'd like to try, too. Bretonnian, Tilean, Daemon, Kizlev, Gnome.