1. Half-life 1 & 2 and it's various mods (CS beta 5.2 through 1.6 mainly)
-Spent more of my life playing this game than any other. It was so revolutionary at the time! I was part of the UK CS scene, playing at the top of the British game for almost 6 years. No other shooter has ever quite captured the 'feel' of Half-life in terms of movement and weapon mechanics, except maybe Halo.
2. Eve-Online
-I don't play this anymore. It was arguably responsible for me getting a less than spectacular 2:1 in Chemistry at Uni! Utterly amazing game though. The rest of the MMO's just don't come close in terms of politics, PVP, risk vs reward, comradery and longevity. WoW and all the other generic elf/orc beating titles are just dogshite in comparison. "Long live the fighters!"
3. Civilization (mainly 4 once patched and with expansions)
-As a purely single player experience this game is unsurpassed. So much complex strategy involved, but once mastered it's just incredibly rewarding!
4. Fallout series
-Mainly 1 & 2 of course. I'll include the whole series though as heavily modded Fallout 3 and NV are pretty damn good.
5. Football Manager series
-For the same reasons as Half-life & Civ. I've spent so much time playing these games its unreal.
6. Xcom (UFO:EU, TFTD & Apocalypse)
-I've sunk many hours into these gems, especially Apocalypse. They don't make em like they used to!
7. Morrowind
-The other TES games don't come close to this masterpiece.
8. GTA
-Every single game in this series has been amazing, without exception. San Andreas was probably the pinnacle and if the rumours surrounding GTA V are to be believed then it's going to be more like San Andreas than the other games!
9. Halo series (mainly 1 & 2 because they came out on PC)
-There's still no rival in terms of a tactical FPS imo. The balance of weapons and the ability/dedication required to play these games on 'legendary' makes them hugely enjoyable.
10. Thief series (mainly 1 & 2)
-Sneaking has never been better and given the way things are going, I doubt it ever will be. :/
Honourable mentions for Deus Ex, Baldurs Gate/NWN & System Shock 2. Of course there will be games I've forgotten about too!
Edit: Damnit, forgot about STALKER! That's somewhere in my top 10.
Edit 2: Dark souls gets a mention as the best console game I think I've ever played.
Edit 3: Damnit, forgot about the X series. Mainly X3, Single player Eve!