Probably:
1. Pokémon Red/Yellow/Gold - I spent countless hours playing and replaying them.
2. Golden Sun/Golden Sun: The Lost Age - The same. Sometimes I would turn my GBA on just to listen to the main theme. =D
3. World of Warcraft - For obvious reasons.
4. Super Mario Land 2 - Childhood + Game Boy + Super Mario Land 2 = Happiness.
5. Final Fantasy X - The only game in the series I've finished more than once.