I'm too lazy to post for each system but SSBB (assuming it's better than SSBM) or GTAIV (assuming it's more fun than III)

Super Smash Brothers Brawl. It will be endless fun.

SSBB

iam sure playing this game online will be great :)

Oblivion. I now have over 600 hours into several different characters...

I'm not sure if this is what OP intended his thread to be, but if I had to pick only one game I could play for the rest of my life, I'd probably make it a MMO. I'm not into the genre at all, and particularly despise World of Warcraft, but I know that would be the best pick.

Out of console games I'd probably choose SSBB for its humongous replayability. If I had to pick one for each console, then either Halo 3 or the Orange Box for the 360, and probably GTA4 or Little Big Planet for the PS3.

It has to have Replay value I got it DOA Xtreme 2

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 ;)