PS3: SUPER RUB-A-DUB, THAT IS ALL.
No but seriously :P Umm so far I'd have to say:
PS3: Warkhawks, so much variety, and updates!
PS2: Final Fantasy XI :D, Grinding/Replay Value/Online Community :D
PSP: Final Fantasy Tactics, Disgaie(sp), or Castlevania Chronicles. (all rpg's that offer me lots o' grinding :D )
PC: World of Warcraft, I don't like this game too much but I LOVE grinding :D
360: So far I'd have to say Dead Rising, just because I love killing zombies, but I didn't really like that game much... meh it's at least replayable.
Wii (if I had one) SSBB me thinks, I liked the last two and honestly who doesn't want to pwn some noubs online ;)
From 0 to KICKASS in .stupid seconds.