1.
Baldur's Gate 2 - First game that really captured me and led me to spend altogether far too much time neck deep in it's world. The words "Coo! Come with me then!" in that distinctive voice will stay with me forever.
2.
Dark Age of Camelot - I considered mentioning WoW since it's almost ubiquitous with MMOs these days (despite a thriving FTP MMO market lurking around), but the MMO that really means the most to me was DAoC. I adored the open PvP: relic raids, keep assaults, open field gank groups, gate camping, etc. Eventually housing, battlegrounds, and more in-depth PvE content started competing for time, and that was (mostly) great, too. My best MMO memories involve massive amounts of people (and lag) assaulting or defending keeps on a scale that no one has really done well since.
3.
Diablo 2 - The "essential reading" of action RPGs. The frantic and sometimes unfair combat mixed with a somehow engrossing world is a combination that few ARPGs have even approached since. It was also my introduction to griefing, scams, and soul-crushing bugs (Anyone that ever had their inventory exploded knows what I'm talking about). Even cow runs! Man, that was actually fun grinding, especially if you got a lightning cow (Man, fuck that lightning modifier).
4.
Civilization 4 The worst part about Civ4 was whenever I played it, the rest of the day (into the night) was just gone. I dont think I ever played this game in a spurt shorter than 4 hours. So engrossing. Im not especially a fan of the end-game management overload, but having games where its you vs. the world for hundreds of years is an experience not found anywhere else. I do like Civ5, but Civ4 is pretty definitively the peak for me.
5.
Portal Just incredible. The fresh mixture of humor and brain bending puzzles was the perfect game for me. I should probably nominate The Orange Box in whole just from how many damn hours Ive spent on that single purchase.
6.
Team Fortress 2 I loved this game when it came out as a unique mp shooter experience. I probably spent around 20 hours around launch, then set it aside for other games. And then hats happened, and I went and rediscovered my love of the gameplay plus a crushing addiction to wearing silly hats while shooting dudes. The entire presentation of the game from the marketing and promos to the in-game character chatter to the perfect kill screen just makes for an extremely pleasant fps.
7.
Age of Empires 2 Age of Kings [TWO POINTS] Fucking damnit this game needs a gog.com release or something. I LOVED playing the multiplayer with friends or online. Castle clumping, or bullshit Mongols, or my personal favorite Goths, it all made for a great game. If I could find my discs, I think Id play this game now if I could.
http://www.microsoft.com/games/age2/ is a soul-rendingly sad site/sight.
8.
Star Wars: Knights of the Old Republic They had this character named HK-47. He was a pretty good character. Its also a Star Wars RPG done right, with an inventive story and interesting combat.
Some honorable mentions:
Torchlight Somehow I really liked this game (despite the final boss taking upwards of an hour and a half for me).
Warcraft 2 Sidewalks killed it. The fact that I own the MAC version of this game is pretty tragic every time I rediscover those discs, too.
Just Cause 2 Insane game (with bolopatch for infi ammo, but not infi health), but not a PC-exclusive, so I didnt want to mention it.
Half Life 2 A great game, but Im not the biggest shooter fan in the world. The atmosphere was really one of a kind, though.
Dragon Age: Origins Ill be honest. DA2 has really tarnished my memory of this game, I think. Despite that hanging over it, I recall really enjoying this game, as well as every single DLC and the expansion. Actually, its just me being bitter.