Final Fantasy Tactics. Seriously, if you said I could only play one game for the rest of my life, I'd pick FFT without a second thought. The game-play is so addicting and perfect. It makes me sad beyond belief that Square has never gone back to the formula depite the game probably having sold billions of copies by now. The challenge of getting all the hidden items out of Deep Dungeon is one of the most frustrating and rewarding things in gaming. Just writing this makes me want to go complete another perfect save.
More recently:
Borderlands & Borderlands 2. I've beaten both games many times on many levels of difficulty. I could probably play through them in my sleep. General Knoxx's Amory, the DLC from the first game, I ran it so often that I had it down to at science.
Might & Magic: Clash of Heroes. Just beat it again a couple of days ago. It has a nice little RPG story but the combat is so fun and inventive.
AudioSurf. Pretty much run through any new music I get or when I rip an old CD. Played some Led Zep songs last night.
