Mmm AVP is great online. Being able to be like, either 3 is what makes it so fun, because each one has their own strengths.
But the game that really get sme going is Rainbow Six 3: Raven Shield online. If you've ever been outside and played at least one game of manhunt, you get the same feel with this game. You hide behind boxes in the dark waiting for the enemy to come, then pop out, shoot him, clear the area. The rest of the team is dead, it's just you. You slowly climb up the stairs into the attic where you see the final enemy walking in the other direction, and you take the shot. Horrah! Online nerds praise you!
That game is so... friggen good