Medieval 2 Total War
SimCity 4
Oblivion
Mass Effect 2
Rayman 2: The Great Escape

I could try to think harder but this makes a fine list

1. The Elder Scrolls III: Morrowind
2. Half-Life 2 (includes Episodes 1 and 2)
3. Mass Effect 2
4. Call of Duty (the first one)
5. Diablo II/Baldur's Gate II (It's a very hard decision, so I'll just say it's a tie.)

It's not that, it's just because people refuse to accept that games could possibly be better than their personal favorites. 10 years from now, Dragon Age: Origins will have replaced all of the Baldur's Gate mentions.

1. Half-Life 2 and it's episodes
2. Half-Life
3. Fallout 3
4. Quake 3
5. Homeworld 2

1. System Shock 2
2. Ultima Online
3. Counter-Strike
4. Left 4 Dead series
5. Age of Empires 1 and 2

5. 9Dragons
4. Rappelz
3. Need for Speed: Carbon
2. Ragnarok Online
1. Warcraft 3

Starcraft ( I only got this recently and am amazed by how well it has aged!)1
Orange Box/TF2 2
cnc series( cant pick just one!) 3
warcraft 3 4
Baldurs Gate 1( I just cant find the time for the second one :S) 5

1- Civil 2
2- Civil 4
3- Age of Empires 2
4- Simcity4
5- Call of Duty2

And yes, older games are dominating cause Pc gaming got a boycot from devs, but civ 5, Diablo 3 and Starcraft 2 gonna change that view. Ask the same question(fav 5 pc games) in 2 years and you'll see.

5 - Age of Empires II

4 - Command & Conquer: Red Alert.

3 - The Battle for Middle Earth. (Why isn't the awesomeness of this game acknowleged?)

2 - Warcraft III

1 - Starcraft

I did ask for what people themselves think. Not the highest rated games by critic scores. Or highest rated games by user score.