Tricky one. Obviously Icewind Dale and Baldurs Gate games should be on this list, but not played them in a very long time and don't think I've played through all of them either.
The first Tropico game and the Children of Nile games are something I keep on coming back to again and again. So these obviously would be very high on the list. Then again the more recent Tropico (3&4) games are more polished and might be the ones I play next time I need Tropico fix.

Of the more "modern" games I'd say Witcher, VVVVVV or Hotline Miami. Can't say Witcher 2 yet because haven't gotten around finishing it. No need to explain why Witcher is a great game I suppose. VVVVVV hits the C64 nostalgia nerve here with its retro visuals and amazing soundtrack (which I've bought several times). Hotline Miami is somewhat similar to VVVVVV for me, except for more the messed violent theme naturally. The top-down perspective works really well as well. Can't wait for Hotline Miami 2!

edit. Just a second after clicking Send it came to me! Deus Ex, the single greatest PC game ever made. Very deep and very playable still today, with the high-res pack and the fixes that is. Will be interesting to see how the third one will be. Assuming they make one. Not counting the recent mobile game, which I havent gotten around playing yet.

Going to go with Under a Killing Moon, that is the game that really sparked my love of Tex Murphy and thankful to GOG for that.

*Mandatory Tesla Effect Plug* lol

Now I am imagining a giant electric socket!

Heroes of Might and Magic 3.

You guys know nothing.

HEY! I may not be very smart but I'm stupid!

Third? Don't you mean fourth? Are are you ignoring Invisible War? (I know it wasn't as well received as DX or Human Revolution, but it was eminently playable!)

Doesnt have to be "only" on GOG or you only played their version. I just want it to be available on GOG for my own selfish and vicarious need to play other peoples favorites.
I tried, I really did, but I cant figure out what this is...

there are a few that I always come back to:

Stonekeep
Sanitarium
Blood
Little Big Adventure 2
Legend of Kyrandia book 1
Shadow Man
...

just thinking about these games makes me want to play them again :) only for a short time, mind you, as when my nostalgia meter is filled, I quit and I never finish them again...

is an indie puzzle platformer with metroidvania elements known for its high difficulty (I didn't find it all that hard, at least if I don't try 100%-ing it, the basic adventure is fairly easy) and trial-and-error old school approach. It's a game by Terry Cavanagh, of [url=http://www.gog.com/game/super_hexagon]Super Hexagon fame.

[EDIT] Also: to any VVVVVV fans out there, I highly recommend you go play the disturbingly awesome suteF, a freeware game by Ted Lauterbach.

Arcanum

Planescape: Torment for sure.

I know its a bunch of peoples favorites, Planescape bored me to tears. And I love the old Gold Box D&D games, Baldurs, Icewind. I just couldnt go with the storyline at all. Would have been a better movie in my mind. Again, i likely didnt give it a fair shake but it lost me so quickly. BG2 is another that almost lost me, the whole "who/where am I" story line stuff never appealed to me in a D&D setting.

Arcanum is definitely one I plan on trying, purchased at some point just havent gotten around to installing it.

STALKER series