As I cannot decide which I prefer to another, it's easier to simply but them in alphabetical order.

Baldur's Gate
Baldur's Gate II
Icewind Dale
Icewind Dale II
Planescape: Torment

Yes, I was disappointed to find Verdun is multiplayer only as well.

-Go
-Fallout 1 and 2
-FTL
-Alpha Centauri / Other Civ games
-Stars! / Other space 4X games

Borderlands
Creeper World 3
Skyrim
Psychonauts
Sid Meyer's Alpha Centauri

Simple:

Gothic 2 + Expansion
Warcraft 3 + Expansion
Monkey Island 3
Gothic 1
Titan Quest + Expansion

1: Skyrim
2: Saints Row 3
3: Dishonored
4: Metro (both because its a contiguous story)
5: Infamous 2

Fallout
Fallout 2
Pool of Radiance
Lunar Silver Star
Disgeae

5. Baldur's Gate II: Shadows of Amn
4. Xenoblade Chronicles
3. Might & Magic VI: The Mandate of Heaven
2. Baldur's Gate
1. Super Metroid

In no particular order:

StarCraft
Fallout 2
Heroes of Might & Magic 3
Unreal Tournament
Diablo 2

honorable mentions:
Abe's Oddysee and Exoddus
Half-Life
Quake
Quake 3 Arena
Supreme Commander
Railroad Tycoon 2
Call of Duty
Dawn of War
Anno 1503
...and many more...

Im gonna cheat and include series, cant really pick a fav one:

Marathon Trilogy
Myst Series
Stalker SoC/COP
Deus Ex
Infinity Engine Games (ID a bit less)

... yep. and just for fun:

5 favourite games played over the last 2 years:

Witcher 3 (series could be in my all time fav list as well when i think about it)
Dishonored
Pillars of Eternity
Bioshock Infinite
Deponia Series

Ah, Deus Ex, the best game of all time I agree. Seemingly has the perfect amount of fun videogame mechanics and environment puzzles crossed with first person action and spy story.

No One Lives Forever, nearly mentioned it. It's a brutally tough stealth game in there if you want to play it like that, if anyone didn't know that. Love the cool little gadgets and training rooms. Twice the game the second one is.

Thief 1
Diablo 2
Morrowind
Sacred 1
Soul Reaver 1

Fallout New Vegas
Deus Ex
Jedi Outcast
Jetpac
Metroid Prime 3

In no particular order:

Chrono Trigger
Final Fantasy VI
Final Fantasy XII
Silent Hill 2
RollerCoaster Tycoon

Chrono Trigger
Xenoblade Chronicles
Zelda Ocarina Of Time
Undertale
Super Mario Galaxy