At the moment I'm having a bit of an obsession with the classic Sonic games, so Sonic 3 & Knuckles wins right now. Otherwise, it would have to be Twisted Metal 2 (wow, I feel old xD).
If you mean my favorite game of all time, I can't pick just one. Some of my all-time favorites include (in no particular order):
-Kirby Super Star (and its DS remake)
-Super Mario Galaxy
-Pokémon Gold/Silver/Crystal
-Sonic the Hedgehog 2 (Genesis)
-Super Smash Bros. Brawl
-NiGHTS into Dreams... (the original Saturn one, not the Wii sequel)
-Super Mario World
-EarthBound
My favorite games include:
-Doom
-Doom 2 (As well as Plutonia Experiment and TNT Evilution)
-Doom 3 (and RoE)
-The Elder Scrolls III: Morrowind
-Golden Sun (and Lost Age)
-Super Smash Brothers Brawl
-Zone of the Enders 2
-Call of Duty (3, 4, 5)
-Left 4 Dead
-Guilty Gear XX (and Accent Core)
-Pokémon Diamond