Oh gosh, it's so hard to choose... It's like asking a parent to pick one of their children! XD Here are my votes, in no particular order:
The Elder Scrolls 3: Morrowind (with Oblivion real close behind, but I had to pick one and the island of Vvardenfell is just too rich to take second place)
Zork: Nemesis (one of the greatest adventure games of all time, with a haunting atmosphere and an unforgettable story)
Team Fortress 2 (considering all the hours I've put into this game, and how much fun I've had with it, and the great bunch I've met on it, I'can't *not* put this one here)
The Curse of Monkey Island (the debate raged internally between this one, which I loved to bits, has voicing and a kickass soundtrack, and LeChuck's Revenge which was the first one in the series I played. In the end, CMI won out)
Baldur's Gate 2: Shadows of Amn & Throne of Bhaal (put both, because you can't separate a game and its expansion which is, in this case, more like the final act; put this one because it must be first RPG I ever played, like *ever* and thanks to an active modding community, it's still totally playable 10 years later)