Ouch this is impossible and makes my head hurt, but I gave it a shot anyway. I'm sure these will be rotating throughout my mind forever, and they're always near the top. I think they've all been my absolute favorite on different days.
1. Donkey Kong (1981, Arcade)
2. Tetris (1989, Game Boy)
3. Wii Sports (2006, Wii)
4. StarCraft (1998, PC)
5. Katamari Damacy (2004, PS2)
Honorable mentions:
X-COM: UFO Defense (PC)
River City Ransom (NES)
Super Mario Bros. (NES)
Super Mario Bros. 3 (NES)
Legend of Zelda: Link's Awakening (Game Boy)
Portal (PC)
Doom II (PC)
GoldenEye (N64)
It was really hard not to put those in the top 5 too.