Civilization II, Scenarios, Fantastic Worlds. Such an awesome game.
Civilization I
Railroad Tycoon I
Panzer General I
Panzer General III - Scorched Earth
B-17 Flying Fortress
Age of Empires I
Command & Conquer I
R.E. Lee - Civil War General
Armada 2525

A few off the top of my head:

Age of Mythology
Assassin's Creed 2, 3, Rogue, and Black Flag
B-17 Flying Fortress The Bloody 100th
Castlevania Lords of Shadow 1 & 2
Dragon Age 2 and Inquisition
Far Cry 3, 4, 5
Ghost of Tsushima
God of War series
LA Noire
Like a Dragon Ishin!
Mass Effect 1, 2, 3
Medal of Honor Airborne
Myth series
Red Dead Redemption 1 & 2
Second Front
Task Force Admiral
This Land is My Land
Ultimate Admiral Age of Sail
Wolfenstein 2009


Games I'd like to see return to GOG:

For the King
The Long Dark

Age of Empires 1 & 2
Age of Mythology
The Cave
Contrast
Diablo 2
Discworld
Dune, Dune 2 & Dune 2000
FAR: Changing Tides
Freelancer
Neyyah (new Riven-like game)
No One Lives Forever 1-2
Oxygen Not Included
Prey (2006)
Quantum Conundrum
Rise of Nations
Subnautica
The Talos Principle 2 (still under development)
The Turing Test

nvm

Lemmings
NOLF
Spyro the Dragon Reignited
Grand Theft Auto 1

Assassin's Creed 2
Assassin's Creed 3
Assassin's Creed: Black Flag

Far Cry 3
Far Cry 5

Subnautica
Subnautica: Below Zero
The Long Dark (bring it back please..)

7 Days to Die

Resident Evil 7: Biohazard
Resident Evil 2 (Remake)

DOOM 2016

Euro Truck Simulator 2

Just Cause 3

Satisfactory

(unlikely, but still..):
GTA 3
GTA Vice City
GTA San Andreas
GTA 5
The Last Of Us

Boyfriend Dungeon
Buck Rogers: Countdown to Doomsday & Matrix Cubed
Cattails
Cattails: Wildwood Story
Cthulhu Saves the World
Danganronpa (series)
Discworld
Discworld 2: Mortality Bytes!
Dune
Dungeon Master
Dungeon Master 2: The Legend of Skullkeep
Four Last Things
Hustle Cat
King's Bounty (as in that very ancient DOS era game)
Persona 3
Please, Touch the Artwork
Stray
The Caligula Effect 2
The Room (series)
The Sims Complete
Uncharged Waters
Viscera Cleanup Detail
Wizardry 1-5

(Top Priority) Games that would light up my day if they appeared on GOG and would prompt me to instantly buy them with a big grin on my face (in rough priority order):
1. Myth trilogy (The Total Codex, including The Fallen Lords, Soulblighter, Chimera, Worlds, The Wolf Age, and the extra maps, plugins, and content bundled with The Total Codex release)
2. Black & White series
3. Command & Conquer series
4. Fable series
5. Vietcong 1 & 2
6. You Don't Know Jack Classic Pack
7. Halo series
8. GTA series
9. Max Payne series
10. Marathon trilogy
11. Diablo 2 + Lord of Destruction
12. Warcraft 3 + Frozen Throne
13. Crash Bandicoot series
14. MediEvil series
15. Alien vs. Predator 2
16. COD series
17. Damage Incorporated
18. Discworld series
19. Starcraft & Brood War
20. Jak & Daxter series
21. Burnout Paradise
22. The Sims series
23. Last of Us
24. Tekken series

High Priority (I may or may not instantly buy, but it would be tempting):
25. The Thing (2002)
26. Killing Floor series
27. Left 4 Dead series
28. Half-Life 1 & 2
29. Future Cop LAPD
30. G-Police series
31. Twisted Metal series
32. Crysis series (GOG is missing 2 & 3)
33. Evil Dead series (Hail to the King, Fistful of Boomstick, & Regeneration)
34. Prey (2006)
35. PROTOTYPE series
36. World of Warcraft (A pipedream, I know, but I'd only want to dip in occasionally for nostalgia, so it doesn't justify a subscription)
37. Resident Evil series (particularly 4)
38. Silent Hill series (particularly 2)
39. Gears of War series
40. Enemy Territory: Quake Wars
41. Mass Effect 2
42. Enslaved: Odyssey to the West
43. Bulletstorm

Good old games that should be on GOG but perplexingly are not (which I want mostly for nostalgia purposes):
44. Civilisation 1 & 2
45. Lemmings series
46. Prince of Persia 1 & 2: The Shadow & The Flame
47. SimAnt
48. Crystal Crazy, Quest, & Raider

Many of the games I'd like to see here have already been listed, so I won't repeat them.
Something that was not mentioned and that I feel is missing on GOG, are some really classic series in the management building genre (games like Rollercoaster Tycoon), such as:

Zoo Tycoon
Jurassic Park Genesis (or similar series using the JP/JW IP)
Planet Coaster

-Aeterna Noctis
-Age of Empires 1, 2, 3 Remakes
-Age of Mythology
-Battle for Middle Earth 2
-Castlevania: Symphony of the Night
-Dark Messiah of Might & Magic
-Dino Crisis series
-Dragon's Dogma 2
-Empire Total War
-God Hand (PCSX2)
-GTA series
-Half Life series
-Halo: The Master Chief Collection
-Heretic 2
-LEGO Star Wars: The Skywalker Saga
-Might & Magic Legacy
-Medieval 2 Total War
-Napoleon Total War
-Resident Evil 2, 4 Remakes
-Resident Evil Revelations 2
-Rome Total War
-Shogun 2 Total War
-Vampire Survivors
-Warcraft 3

Castle of the winds 1 and 2. Windows 3.1 lite rpg
Solar winds. Awesome dos space combat game.
Dare to dream. Windows 3.1 point and click.
Silverball. Dos Pinball game.
Earthbound. Snes rpg.
Overkill. Dos shooter.
Stunts/4d driving. Dos stunt base racer.
Alpha waves/continuum. Polygon dos puzzle.
Final fantasy 1-6 pixel remaster.
Zzt. Ascii bases puzzler.
Megazeux. Like zzt but slightly more advanced.
Champ series. Solid dos ports of arcade games.
One must fall 2097. Dos fighting game with robots.
Zone 66. Dos shooter.
Ken's labyrinth. Dos fps.
Seek and destroy. Dos shooter.
Traffic department 2192. Dos shooter.
Vinyl goddess from Mars. Dos platformer.
Emperor battle for dune. Windows rts.
Command and conquer red alert 2. Windows rts.