In no real particular order:-
Max Payne 2: The Fall of Max Payne
Deus-Ex
Baldur's Gate 2: Shadows of Amn
World of Warcraft
Unreal Tournament 2004
Halflife 2
Rise of Nations
Battlefield 2
Mass Effect
STALKER: Shadow of Chernobyl

Modern Warfare 2
Modern Warfare 2
Modern Warfare 2
Modern Warfare 2
Modern Warfare 2
Modern Warfare 2
Modern Warfare 2
Modern Warfare 2
Modern Warfare 2
Halo 3
Oh, wrong forum.

Civilization IV
Baldur's Gate 2
Diablo 2
Arcanum
Deus Ex
Morrowind
Psychonauts
The Witcher
Prince of Persia: Sands of Time
Divine Divinity

1. Baldur's Gate 2
2. Rome: TW
3. Civ 4
4. Crysis
5. Football Manager 2010
6. Dawn of War 2
7. Battlefield 1942
8. Warcraft 3
9. Rise of Nations
10. Half-life 2

Surprised there's no Hidden & Dangerous 2 or SWAT 4 or Rainbow Six love, tactical shooters are awesome! xD
That said, even I'd forgotten about them ¬_¬

In no order
Star Wars Knights of the old republic
Sacred 2
Mass effect
Dragon Age Origions
Civilisation 4
Elder scroll Oblivion (fun when you add peoples mods to it changing the game into something 100x better)
Supreme Commander
Saints Row 2
Audiosurf

Team Fortress 2 (Multiplayer FPS)
STALKER (Singleplayer FPS)
Diablo 2 (Dungeon Crawler)
Cave Story (2D Platformer)
Plants vs Zombies (Tower Defense)
Star Wars Kotor (WRPG)
Persona 4 (JRPG)
Resident Evil 4 (Survival/Action Horror)
Super Smash Bros Melee (Fighter)
Company Of Heroes (RTS)

no order
- hitman: blood money
- morrowind
- neverwinter nights (and expansions)
- deus ex 2
- batman: arkham asylum (played this on xbox tho, but it is ... woah)
- vampire: the masquerade - bloodlines
- dawn of war
- defcon
- painkiller
- half life 2

The Sims 2
Spore
Oblivion
Fallout 3
Empire Total War
Dawn of War 2
Thief 3
Invisible War
and I can't think of any others right now...

Oh you little pirate.

To tell the truth, it is impossible to list just TOP 10 games. I tried, and I was able to write such a list, but five more games were so close that I'm not going to leave them out.
Civilization IV
Witcher
Gothic II
Fantasy Wars
Dangerous Waters
Crusader Kings
Combat Mission 2
Medieval II: Total War
SimCity 4
Space Rangers 2
King's Bounty: The Legend
Mass Effect
Deus Ex
Baldur's Gate II
Dragon Age: Origins
Knights of the Old Republic
EDIT: I forgot KotOR, so make that six more games that I'm not going to leave out.

My issue is that there are PC-exclusive games, there are still the ports and make the decisions much harder.
I'd have said
"YOUR top 10 PC-exclusive games", it would've lessen the number of choices and I guess that's what OP wants.

It's not PC-exclusives. Having a PC port, you are OK to add it here: you can add a game like Resident Evil 4 which has a PC port and console versions.
What you can't list is console-exclusives games, like Brutal Legend for PS3 and X360 only.

1. Deus Ex
2. ArmA 2
3. Half-Life 2
4. Metal Gear Solid 3
5. The Sims
6. Civilization IV
7. Dragon Age
8. Team Fortress 2
9. Morrowind
10. STALKER