I love my 360. I play it a ton. Lots of Halo 3 and Battlefield Bad Company. However there are three games that I play a lot more than any other game. Two of them are on the PS2 and one is for the PC. The games are old and yet are still very fun to play. They are

1, Killzone - possibly the most underrated shooter ever. Gamespot gave it a score of a 6.9, and I find this game to be among my favorites. I still have a blast playing the story mode, and every weekend I play this game with my friends. 

2. Star Wars Battlefront 2 - once again a game I play with my friends a lot. There is a lot of replayability with the conquest mode and the story mode is solid. 

3. Age of Empires 2 - the greatest RTS of all time. I have been playing this game for 8 years and I have yet to be bored of it. I love playing as different civilzations and mastering each one's tech tree. (By the way the 3rd Age of Empires sucks)

Even though I love and play my 360 to death, the three games that I play the most have been out for a REALLY long time.