I have a TON of favorite games, so I'll just list the five that came to mind first:
1. Team Fortress 2: over 600 hours clocked in less than a year. It's a brilliant game. a gem in the greyish-brown pool of stagnant water that is the FPS market.
2. Red Dead Redemption: it's better than GTA4. Deal with it. The characters are wonderful, the story is rich, and the ending is one of the most touching things I have ever seen in a videogame. Manly tears were shed.
3. Batman Arkham Asylum: you don't PLAY as Batman, you ARE Batman. Can't wait for Arkham City.
4. Spiderman 2: until Arkham Asylum came around, THIS was the best superhero game of all time. Even after I beat it I still spent hours swinging around New York, kickin ass, slingin webs, and takin names.
5. Pokemon: for me it's either Silver, Crystal, Soul Silver, or White version. Silver was my first videogame EVER. The fact that Pokemon is still so enjoyable for me after having played it for 10 years is quite a feat, and deserving of a spot on this list.