A few years ago, I'd have said Metal Gear Solid, but that series is dead to me. So many shitty sequels and offshoots, it's pathetic. The original game is still good, but I don't hold it in high esteem any more.
Grim Fandango - Never heard of
Half Life - Never really liked (I don't like HL2 either, sue me)
Gran Turismo - Spent A LOT of time on this game. Loved it.
Starcraft - Played quite a bit, but I suck at RTS games. I don't really care for it anymore
Legend of Zelda: Ocarina of Time - Loved it, but never beaten it. I want to go back and play through it again someday
Metal Gear Solid - Reasoning above
Fallout 2 - Played fallout 1 & 3. Didn't care for them, doubt I'd care for Fallout 2.
Mario Party - Meh.
Resident Evil 2 - Cool game. First and only resident evil game I beat. But, I don't remember much from it.