NES: Super Mario Bros (probably)
SNES: Donkey Kong Country 2
Sega Genesis: Sonic 2
GameBoy: Pokemon Red
N64: Super Mario 64
Gamecube (never owned but Wii backwards compatability): Wind Waker (probably)
Wii: Super Smash Bros Brawl
3DS: Ocarina Of Time
Playstation: Final Fantasy VII (probably)
PS2: This is probably my hardest because I don't know if I can decide between Kingdom Hearts, Metal Gear Solid 3, or Shadow Of The Colossus. Seeing as how I've been on such a KH kick lately I'll go with that one although I think MGS3 would win any other day.
PS3: Batman Arkham Asylum
PSP: Kingdom Hearts Birth By Sleep