Amazing how many people answer SSBB for the Wii. If the game winds up actually be less than perfect (sacrilege, I know) Nintendo might be under siege of an angry mob with torches and pitch forks.
Looking around the man cave, if I had to choose one for each platform from stuff I actually own:
PS3 - Haven't found *the game* yet, hopefully it'll be UT3. Until then I guess I'll settle for Warhawk, but I'm not happy with that answer.
PS2 - SSX3
Wii - Wii Sports
N64 - Mario Kart 64. Maybe the toughest choice, but I need a racer somewhere.
DS - Pokemon Pearl
GBC - Tetris DX
3DO - Star Control II
PC - Warlords II Deluxe (or Warlords 3 if I'm not allowed to run DOS/Win 3.1).