I didn't really enjoy KF and Borderlands. Found them both pretty repetitive, and levelling systems in FPS always seems to be pointless grinds.
I'd go for lead and gold, since it doesn't force you to play the same class over and over
Another vote for Killing Floor here!
I'm getting fed up with l4d, and BC2 is still broken. So I'm looking for a good team based game to play online, preferably with smaller teams, larger games just become a disorganised mess and are no fun
The Hidden. Half life 2 mod.