1. online pc: Diablo 2 and Hopefully d3 if they decide to put it out
2. offline pc: gta 3
3. PS2: gta san adreas
4. GC: legend of zelda{collecters edition}:majoras mask beast how they made it for 2 systems
5. xbox360: crackdown
Hands-down. The Halo series. Everyone of them had great multi player (even the first even though there was no x box live for it. Internet on the computer version was fun.) Even the Halo 3 beta was friggin' awesome. And that was just the beta. Thanks.
~xCOLDxKILLAx