Hmmmmmmmmmmmm. Good one. Basically it's just going to be the games i've spent playing the longest - as i assume that i'll be able to replay them for even longer...
In no particular order then:
Pokemon Red/Blue
This game would be good because you can replay it so many times with different teams of pokemon, and it changes each time you play it. The first few times i played it, i never took any notice of stats or any of the complicated stuff, but there's so much more to this game that comes out of repeated plays.
Spiderman 2
I LOVE this game. There's something so calming about just swinging around Manhattan, occasionally solving crimes, or just leaping off tall buildings web-shotting to safety. The story is fun, but mainly just the free-roaming makes this playable for a long time.
Just Cause 2
Again, the story for this is a bit crappy, but it's the huge world full of stuff to explore that i loved so much. Just messing around crashing stuff into other stuff, then grappling onto a enemy helicopter, punching out the pilot and then flying away. Badass.
Super Smash Bros
The original for the N64 - for some reason i preferred the physics and fighting in the original that i think has just been getting worse ever since. I like Melee, but i thought Brawl wasn't as good. Anywho, I love me some Smash Bros.
Halo
Depends on whether i'd be allowed to play online multiplayer, and there would foreverbe people to play against? If so, it would be some iteration of Halo. Don't mind which, although i've spent longest playing Reach, but i'd just want some awesome multiplayer carnage to play for the rest of my life.