Duke Nukem 3D

Because its the absolute very best at what it does.

These hypotheticals always seem quite silly to me.

The first Age of Wonders, I can play it all the time

Somebody hasn't prepared for the zombie apocalypse.

No, I'm preparing for the REAL shit that will go down eventually probably maybe in our lifetime. Video-gaming during zombie times? probably not

Space Rangers 2 Reboot/HD.
Crazy amount of game modes.

For me it would be The Complete Ultima VII (technically two games with one add-in each but sold as one).
I originally came here when they had only the Ultima Underworld games and stuck around for the mere prospect of Ultima VII coming. It's still my favorite game of all time.

Zork

I also would say Fallout (maybe nr 2) because of it's replayability and open world. Linear games would drive me mad after a short time :-)

Saints Row I....oh, wait,
I´ll go with Bloodlines.

Goddamnit, those 3 games have a tie currently:

M&B Warband

Morrowind

Wizardry 8

That game seems to be the very definition of "balls to the wall" action.

From the games that I own on GOG and that my computer is able to handle for the moment...

Risen 2: Dark Waters

Game has pirates, a filthy swearing gnome, voodoo magic, more pirates, and I can make my own grog. MY OWN GROG!! I love the game through and through. As a matter of fact, I will download it now.

It's so good to see that somebody else remembers the Kohan series. If it weren't for my boxed copies I would have sworn I imagined these games, for as many people seem to have played them.

To answer the question, it would probably be Heroes of Might and Magic 3 for the campaigns. I absolutely hate the semi-realistic sprites, but the story is just that good. If I could also keep Heroes of Might and Magic 3 Chronicles, which are pretty much stand alone story expansions, I'd be in heaven.

Doom.

Reason: Mods.