CoD4 is amazing multiplayer. best FPS multiplayer ive ever played.
Bioshock is a hit or miss kind of thing. you either love it, or you can't stand it / get bored quickly, i was the 2nd, got bored fairly quick in the game.
UT3, havent tried, but i presume it is cool, haha, i never got into online UT, but always played it @ lans and was lots of fun, ahah