It's been a while I think since I've seen one of these threads. For me:

1.Blade Runner
2.Warcraft 3
3.Kerbal Space Program
4.Batman Arkham City
5.Dwarf Fortress

edit : I'm no good with thread titles this week :-\

In no order:

StarCraft
Baldur's Gate 2
King of Dragon Pass
FTL
Imperialism

* your (sigh)

Oh, top 5. Let me just quickly and sloppily put a completely-wrong list that would make many of my games angry at me. Not necessarily in order of preference:

1. Super Mario Bros. 3
2. Planescape: Torment
3. Doom
4. Castlevania: Symphony of the Night
5. Command and Conquer: Red Alert

Emperor: Battle for Dune
C&C Red Alert
Warcraft 3
Starcraft 1
Supreme Commander Forged Alliance

Edit: Honorable mention Star Ruler 2 from all the new games.

I are top 5 favorite games are something like this:

Final Fantasy V
Dragon Quest 3 (remakes, not original)
Wizardry 8

Can't think of the rest.

Amnesia:The Dark Descent (or basically everything that Frictional have made).
Magicka Wizard Wars.
S.T.A.L.K.E.R. Call of Pripyat.
Portal2 (for funny co-op shenanigans).
Magicka (Also for co-op shenanigans).

In no particular order:

-System Shock
-Deus Ex
-Thief
-Uridium (C-64)
-Football Manager (C-64)

The two C-64 games, I could play for hours without end, and in case of the PC games I could add (most of) their sequels, too - but then it wouldn't be a Top 5 anymore. :p

Arkanoid
Neverwinter Nights
Baldur's Gate 2
Shining Force
Simcity 4

Myst
The Secret of Monkey Island
Sam and Max Hit the Road
Grim Fandango
Machinarium

Kind of predictable for anybody who knows me, really.

All-time?

Railroad Tycoon 2
Harpoon
BF1942
Independence War 2
Operation: Flashpoint

Honorable mention to Terraria, since I don't think I've played anything else after installation a good while back.

Ever played Sill Life?

Of all time, in no particular order:
Age of Empires 2
Mass Effect 2
XCOM 2
Dawn of War 1 (With soulstorm expansion.)
Kotor 2

Honourable mentions (Again, in no particular order. If this was top 10, these would be included.)
Heroes of Might and Magic V (With Tribes of the East expansion.)
Kotor 1
Supreme Commander: Forged Alliance
Mass Effect 3
XCOM: Enemy Unknown (With Enemy Within expansion.)

Not yet. Own it though, and plan on it. Gotta play Post Mortem first.

Oh right, I forgot that Still life is a sequel. I have to remember to check it out sometime. What made Still Life great for me was the tough but fair puzzles, the rich atmosphere and the story.