For multiplayer, Team Fortress 2 wins hands-down. Get a mic, get a friend, and then go blazin' through amazing custom levels.
For singleplayer, I'm going to say Half-Life 2. The game left me stunned after I completed each game (HL2, ep1, ep2) and I still remember how fu-cking amazing it was, but I can't see myself playing it again in the near future.
But recently I've been playing Far Cry 2, an amazing singleplayer open-world really makes me love it. Not to mention the realism of the guns, the only thing stopping me from loving Far Cry 2 to death is that my computer lags even when it's played all on low, probably with about 45 FPS.
My favorite game would have to be Fable II. It is without a doubt, the best RPG I have ever played. There's the awesome game map, the colorful NPCs, and the character customization was amazing. The new control scheme for battle was far better than the previous title and the skills and abilities were awesome. The new quests were fun, and the wider variety of enemies were amazing and challenging. Overall, I thought the game was amazing, which is why it's my fav.
Ninja Gaiden 2, it always gets me with it's amazing graphics and it's stream of non-stop action. Where if I just feel the urge to beat the living hell out of something, I can, without having walk much until my next intense fight. What could make it better? Probably a story that doesn't make my eyes bleed while I scream "WHY!"
