Yea I'm kinda on a Halo 3 kick right now and have been playing it almost everyday lately, so I'm going to go with that. So many different scenarios to play online multiplayer with, never gets boring.
It all depends on the system in question.
Wii: Soul Calibur Legends
PS3: Soul Calibur 4
360: Halo 3
PC: Call of Duty 4: Modern Warfare
Mac: Guitar Hero 3: Legends of Rock (It's true, it's gonna be on Mac)
DS: The Simpsons Game(OUT THIS MONTH YAY!)
PSP: 50 Cent Bulletproof G-Unit Edition (DAMN YOU KANYE WEST)