There are so many great games on GoG, I can't name my favorite. I'd have to make a huuuuge list! Some of the many favs are Carrion, Webbed, Death''s Door, Horizon Zero Dawn, the Trine series, Kingdoms of Amalur: Re-Reckoning, Desperados 3, Overload, Haven Park, A Short Hike, etc etc etc....

My favorite game on GOG is...

... Dragon's Dogma: Dark Arisen.

Runner up...

Mafia II

blade of darkness
is my fav..

Thief 2

Nothing better than to skulk around and steal everything that isn't bolted down.

I don't like these limitations! Isn't there a bank simulator game? Nah, never mind, that wouldn't be much of a challenge :-P

That would be Pharaoh. It's something I found in here randomly several years ago because I liked the art, didn't know about the game previously. It is now one of my favorite games ever.

A mention goes to Beyond Good & Evil which I rebought here even though I had a perfecly fine disc already.

Gothic would have to be my pick.

It's a tossup between RingRunner, WindForge and SPAZ.

Mods goes a long ways.

So many good games on here I can't pick just one. Some of my faves are:

The Witcher 3
Baldur's Gate (all of them)
Pathfinder: Wrath of the Rigteous
Neverwinter Nights Diamond
Fallout 3
Elder Scrolls: Morrowind
Icewind Dale Games
KOTR games
Civ 4

Considering the literal meaning of your statement, I have to say: definitely War Wind II: Human Onslaught. Not exactly a GOG game, I know, but look at that publisher :P.