This list constantly changes for me. Games tend to move up in rank when I've just played them, because fond memories are fresh in my mind. To be fair and have variety, I'm limiting it to one game per franchise. For the moment:
1: Mario RPG Legend of the Seven Stars (Just replayed it, and have lost count how many times I've replayed it)
2: Zelda Wind Waker (Just replayed it 4th time)
3: Team Frotress 2 (Just finished playing it a few hours ago)
4: Metroid Prime (Only played once, but got 100%)
5: Chrono Trigger (good times... good times. Still trying to get all endings)
Honorable mentions:
Pokemon (Spot reserved for White)
Megaman 2 (Replayed it last month for 3rd time)
Megaman Battle Network 5 (Personal favorite for the ability to control others)
Red Dead Redemption (I have a soft spot for westerns)
Marvel vs Capcom (Spot reserved for 3)