So far this is my findings;
Modern Warfare 2 is the, hands down, worst multiplayer game ever.
Followed closely by Operation Flashpoint Dragon Rising.
Battlefield Bad Company 2 is pretty damn good just as long as you get on a decent server where people actually know how to play the game. And not a server full of people who couldn't even team work there way out of a bag.
Rainbow Six Vegas 2 is good as long as you only play co-op.
Borderlands is solid and fun.
Modern Warfare can be fun until you find the servers with the sad act types.
Battlefield 2 is !@#$% online but Project Reality is awesome online (though that is now coming to Arma 2).
I've not tried many games, would love to play some ArmA 2 online but need to get me new laptop first, and some games are just terrible ideas for online -_- anyone else wanna add?