Here's what I can think of right now.

Call of Duty: United Offensive [Multiplayer] (PC): This expansion pack basically made everything in the game better. I was never able to get into CoD2's PC multiplayer, so I've played this consistently since it came out about 2 years ago.

Super Smash Bros. Melee (GCN): I honestly don't think this game will ever get old for me. I've yet to find a time where I didn't feel like playing a few rounds.

Mario Kart Double Dash!! (GCN): Same as with SSBM. I'm almost always up for a round of racing and red shells.

Thief (PC): The original Thief will always have a place in my heart, and its large and open missions allow for a different game every time.

Super Metroid (SNES): This game is pure 2D bliss for me. That is all.