My most wanted games:
1 Mass effect legendary edition
2 Dragon Age 2 and inquisition
3 Total War Warhammer series
4 Marvel's Spider-man and Miles Morales
5 Assassin's Creed series
6 Shadow and Rise of the Tomb Raider
7 Max Payne series
8 GTA series
9 No one lives Forever series
10 Battle for middle-earth series

Besides what everyone else said, here's my list (and some might be repetitive, didn't look at all lists).
01. Mythos (with some fixed translations too)
02. Hellgate: London (old version all updated AND also the newer Steam-version w/ more content form Hanbit)
03. Alpha Protocol.
04. Fable series....especially including a Fable 2: Complete Edition PC port.
05. Halo: MCC.
06. Entire JRPG Collection of stuff from Square Enix (Final Fantasy series, Dragon Quest series, Chrono series, SaGA series, NIER series, Octopath Traveller series, etc etc).
07. Tales series from Namco Bandi
08. Scarlet Nexus (and its Season Pass)
09. Gears of War: Complete Collection - also especially including missing PC ports like Gears 2,3, and Judgment)
10. Bayonetta series (up to Bayonetta 3)

The Dream - Always-online games/MMO-lites/MMO's turned into offline working games with Bots/AI, LAN support, and also Galaxy support:
01. The Secret World/Secret World Legends (with ALL Expansions/DLC's/Extra content)
02. Tabula Rasa
03. Guild Wars 1 and 2 (with all expansions)
04. Fallout 76
05. Diablo 3 (with RoS and the Necromancer DLC) and Diablo 4.

Ghost of Tsushima
Doom 2016 & Doom Eternal
Deathloop
Returnal
Death Stranding
Persona 5

Basically any and all AAA-game.

In no particular order:

- Vietcong (2003, by Pterodon / Illusion Softworks (2k Czech)) (the first one - I don't really need the second installment)
- Richard Burns Rally (2004, Warthog Games / SCi (Square Enix))
- Hunter Hunted (1996, Sierra On-Line)

Already mentioned:

Ecstatica (1994 + sequel 1996)*
Discworld (1995 + sequel 1996 + Noir 1999)
(Captain) Claw (1997)
Heart of Darkness (1998)
Heretic 2 (1998)
Get Medieval! (1998)
American McGee's Alice (2000)
Heavy Metal: F.A.K.K.² (2000)
No One Lives Forever (2000 + sequel 2002)
Prey (2006)
Wolfenstein (2009)*

So I'll add a Top 10 of games that weren't listed yet:

1. EcoQuest: The Search for Cetus (1991)
2. Lost Secret of the Rainforest (1993)
3. Drakan: Order of the Flame (1999)
4. Kiss: Psycho Circus - The Nightmare Child (2000)*
5. ONI (2001)*
6. Archangel (2002)*
7. Darkened Skye (2002)*
8. The Hobbit (2003, incl. German voiceovers)
9. The Simpsons: Hit & Run (2003)*
10. Dead Man's Hand (2004)*

* out of curiosity, no idea if they're good

Btw, wasn't Legends of Might and Magic (2001) here on GOG one time? And was it truly that terrible? I think I'd have liked to try it, at least.

EDIT: Ah, I see I confused it with Crusaders of Might and Magic, and that one is still here, just never talked about. And apparently Legends of Might and Magic was mainly a multiplayer online game, so maybe not that interesting for a GOG release after all?

Everyone suggesting Halo MCC to be on the storefront seems to forget that you can't even play without an XBOX account, and you can't do multiplayer without the DRM that is the Easy Anti-Cheat. If GOG even think about putting anything Halo, I would've preferred that they put the original PC ports of the first two games instead. With bug fixes, of course.

My personal Top 10 (including optional picks):

1. The Sims 1 & 2 Complete Collections
2. Road Rash (1994? PC version)
3. Best of Windows Entertainment Pack '98 (Chip's Challenge, JezBall, etc.)
4. Zoo Tycoon 1 & 2 (w/ all expansions included in the installation)
5. GTA Top-Down Collection (1, London 1961, London 1969, 2. Bonus points if Chinatown is included as well)
6. Bugdom 1 & 2
7. Barbie Seahorse (no relation to Mattel's Barbie whatsoever)
8. The Sims Medieval + expansion
9. Need For Speed: Most Wanted (2005)
10. Warcraft III Classic Complete Collection

Optional Picks (meaning not a priority):
-The Sims 3 (w/ all DLCs INCLUDING all Store content and Katy Perry DLC)
-GTA 3D Anthology Classic (III, VC, SA, LCS, VCS)
-Need For Speed: Underground 1 & 2
-The Simpsons: Hit & Run
-Spider-Man Activision trilogy
-Black & White 1 & 2
-Command & Conquer franchise up to Generals: Zero Hour
-Total Distortion
-Swords & Sandals: Classic Collection, 2 Redux, and Crusader Redux

1. Portal 1
2. Portal 2
3. Half Life 1
4. Half Life 2
5. Mechwarrior games
6. William Shatner's Tekwar
7. Doom (2016)
8. Doom Eternal
9. Capcom Fighting Collection
10. No One Lives Forever

Unreal and Chrome.

Hmm...
1. Starcraft
2. Portal
3. Hades
4. Diablo II

Total Annihilation
Black & White

But Total Annihilation is on GOG...

https://www.gog.com/en/game/total_anihilation_commander_pack

My Top 5 would be:

1.Oneshot
2. The 3D GTA games
3. Battlefield games
4. The Forest
5 Sons of the forest

1. Black and White 1+2+Addons
2. No One Lives Forever 1+2
3. Comix Zone
4. Fifa 98 WM and Fifa 99 and maybe 2002 WM
5. Virtual Cop 2 and House of the Dead
6. Tony Hawk Pro Skater 2 Classic
7. Age of Mythology
8. DRIVER 1 (not 2 or 3, just 1)
9. N.I.C.E. 2
10. Need for Speed Underground 1+2 + High Stakes + Porsche

And maybe Lost Vikings and Scarface

Dune
StarCraft
Mass Effect trilogy
No One Lives Forever
all Command & Conquer RTS
NES and SNES games (Metroid and Super Metroid in particular)

- Dragon Age 2 and Inquisition
- Hades
- Vampire Survivors
- Tales of
- Persona 5
- Diablo 2
- Total War
- Doom 2016 (I'm baffled this STILL isn't here lol)

Just the first ones that came to my mind