1. Quake
2. Max Payne 2
3. Tetris
4. Far Cry
5. Dead Nation


I chose based on how often I replay.

This is tough for me because different games fit different moods for me. I guess my top 10, in no particular order would be:

1. Baldurs Gate 2 - Probably the best single player game I've ever played.
2. StarCraft (one and two) Good campaigns and best multiplayer ever. I think I have played more hours on this than any other. Before SC2 came out I played with a group of regulars on Hunters Fastest maps. SOOO much fun.
3. Sword of the Samurai - A simple game, but I really enjoyed it
4. King of Dragon Pass - Great lore, immersion, meaningful choices. Surprisingly deep game
5. Master of Orion 2 - Design your own ships and conquer the galaxy, done right!
6. Majesty - A medieval sim game. I loved that you couldn't directly control the hero.
7. FasterThanLight - The first indie game I got into. I love that the expansion was free. Great job! I bought lots of extra copies and gifts.
8. Escape Velocity Nova - Back from my Mac gaming days. It was fun watching the evolution from EV to EV Override to Nova. Plugins are awesome and tons of replayability
9. Imperialism- Originally played Imp2 first and loved it. I love Imp1 even more.
10. Age of Wonders- So much fun. Like it better than HoMM. Campaign is great and races are balanced. I'm looking forward to buying number 3 at some point.

Honorable mentions:

Civilization 4 - Fun, end game is always a slog.
Expedition: Conquistador - Lots of fun, the camping mechanic gets old after a little while. Fun story and combat.
Jedi Knight 2 Jedi Outcast - Lightsabers.... Nuff said. Good campaign and played online with my friends.
Stronghold - A classic. Econ and battles.
The Banner Saga - Good game, great story. Want the other two parts.
ShadowRun Returns - Fun, looking forward to Dragonfall
Banished- Playing it now. I could be great. I'll give it some time.
Sim Farm/Sim Ant - Classics. Probably not as much fun if I played them now lol

*Highfives fellow Tetris player* :-)

^ If you highfive FoxySage use your free hand to highfive me for Max Payne 2 !!

edit: or we can all just lay our hands on tinyE if he's gonna get all up in the middle of this

That and Pinball.

I'd be fine with this. Everyone dogpile TinyE! :p

1. Morrowind - I can't forget how many years I've spent playing and replaying this game, The game is absolutely perfect (except the bugs).

2. Legacy of Kain: Soul Reaver - Where to begin, got this when I was 12, Superb voice acting, amazing story and great gameplay overall. Should be a movie.

3. Doom 1 - Played this at a very young age (18+ huh?), I started with the shareware, even finished the first episode on Nightmare difficulty. I still like it over Doom 2. Modded it to hell even.

4. Unreal Tournament 2004 - For me remains the best Multiplayer / Arena shooter ever made. I still play it now and then.

5. Divinity: Original Sin - Fairly modern to call it one of the best of all time and some people will want to shoot me for choosing it over Baldur's Gate (I like BG too). There's just something about this game that I reallllly reallly like. It could be the setting, the gameplay, I don't know how to describe it. I just like it a lot :p

5. Day of the Tentacle
4. System Shock 2
3. Bioshock
2. Braid
1. Binding of Isaac: Rebirth

Why are you sorry that certain games aren't on Steam?

5 - Fallout: New Vegas
4 - No One Lives Forever 2: A Spy in H.A.R.M.'s Way
3 - Giants: Citizen Kabuto
2 - Deus Ex
1 - The Operative: No One Lives Forever

--Honorable mentions in no particular order--
System Shock 2
F.E.A.R.
Deus Ex: Human Revolution
FTL: Faster than Light
Super Mario World
King's Quest VI
S.T.A.L.K.E.R.: Call of Pripyat

Blade Runner
[url=http://en.wikipedia.org/wiki/Warcraft_III:_Reign_of_Chaos]Warcraft 3[/url]
Fallout 2
Civilization 4
Kerbal Space Program

edit: aaaand I remembered what I forgot : Commandos 2 but I don't know what to replace :P

Well they kinda change depending on mood, but at the moment I would say:

- Spec Ops: The Line
- Planescape: Torment
- Thief 2
- Fallout
- WarCraft III

Hmm, not an easy question!
5. Titan Quest
4. Doom
3.[url= http://en.wikipedia.org/wiki/The_Bard%27s_Tale_II:_The_Destiny_Knight] The_Bards Tale II: The Destiny Knight[/url]
2. Civilization
1. Imperium
Well, that should be my list.

You and some others mentioned Thief, this also is one of the highest rated games on GOG.
What makes it so special?
I have it in my library but did not yet find the time to play it.
On first sight, it looks rather average...

It would because the gameplay makes it special, not the look. Install it and just play the tutorial, that should be enough to get you hooked. Also keep in mind that when the 1st one came out EVERY FPS was just charge in and blast away while Thief actually penalizes you for killing.