Now it's my turn to ask: Why? Just PC gaming or video gaming in general?

Day of the Tentacle, Heart of Darkness, Homeworld, Ignition, Jazz Jackrabbit 2, Need for Speed 3, Re-Volt, Red Alert, Quake 3, Starcraft, Tomb Raider, Worms Armageddon... Good times...

Limited to what you can purchase on GOG:

Alpha Centauri
Castles
Diablo
Lands of Lore: The Throne of Chaos
Master of Magic
Master of Orion
Might and Magic
Quest for Glory
Ravenloft: Strahd's Possession
Veil of Darkness
Wing Commander: Privateer

Gaming in general. I left college and began a career... or a few. Didn't have the time to game. In college we played Nintendo and PC games (to this day I still carry around a copy of Indiana Jones and the Fate of Atlantis). I returned to gaming mid-way through the PS2 lifecycle... and have slowly come back to PC gaming.

I have a pretty consistent top 5 and only one of those is from before 2000, which is Fallout 1. Sam and Max Hit the Road would be in my top 10, and that was also from the 90s.

With the exception of Baldur's Gate II: Shadows of Amn, all my favourite games were released before 2000: chess, nethack, Planescape: Torment, Starcraft, Fallout, Super Metroid, Sid Meier's Alpha Centauri, Aliens versus Predator, King of Dragon Pass, Theme Hospital.

Immediately coming to mind:
Betrayal at Krondor
Might and Magic VI: The Mandate of Heaven
Planescape: Torment
Quest for Glory II: Trial by Fire
Supaplex

Added after looking through what I played:
Transport Tycoon Deluxe
Elite Plus
SimCity 2000

Initially left out but deciding to (re)add when I saw that I had eight, so two more would make a round ten:
Stunts
StarCraft: Brood War

Heroes of Might and Magic 3;
Age of Empires 2*;
Silent Hill 1;
Chrono Cross;
Final Fantasy VIII.

*prefer Definitive Edition

Two excellent games that haven't yet been mentioned:
- Roller Coaster Tycoon
- Gabriel Knight. Sins of the Fathers

On GOG:
Chasm: The Rift
Transport Tycoon
Thief
Heretic
Might and Magic Series
Blade Runner
Diablo
Albion
Theme Hospital
X-Com: UFO Defense
X-COM: Terror from the Deep
X-Com: Apocalypse
Ultima Underworld: The Stygian Abbys
Commandos - Behind Enemy Lines
Commandos - Beyond The Call Of Duty
Shadow Warrior
Total Annihilation
Bio Menace
Fallout 1 + 2

Waiting for GOG release (or least hope, that it will happen one day):
Claw (Captain Claw)
Age of Empires + The Rise of Rome
Dungeon Master II
Spellcross: Poslední bitva (The last battle)
Horké Léto

And more...

Planescape: Torment is my favourite video game of all time.

I'm going to have to go with the obvious option and say Heroes of Might and Magic 3. Is there a bit of nostalgia connected to this? Well yes, certainly (even if I don't really understand why since I was was really, really bad at it when I was young). But I genuinely feel that it holds up in every way to the games in its genre that released after it. Including visually (due to its great art direction) which is very impressive for a game released in the previous millenium to do.

Pretty sure I've posted a list like this before, somewhere, but why not:

1990: Loom, The Secret of Monkey Island
1991: Monkey Island 2: Le Chuck's Revenge, Eye of the Beholder, EOB II: Legend of Darkmoon
1992: Indiana Jones and the Fate of Atlantis
1993: Forgotten Realms Unlimited Adventures, Lands of Lore: The Throne of Chaos, Doom, Sam & Max Hit the Road
1994: Wacky Wheels, Little Big Adventure, Heretic
1995: Albion, Discworld, Full Throttle
1996: Tomb Raider, Quake (but I really only like the 2021 Enhanced version)
1997: (Captain) Claw, The Curse of Monkey Island, Tomb Raider II, Outlaws, Oddworld: Abe's Oddyssee, LBA2
1998: Grim Fandango, Rage of Mages, Heart of Darkness
1999: Planescape: Torment, Discworld Noir, Rage of Mages 2

Most of my favorites nowadays are relatively recent, but man, videogames back in the day were so gripping.

Simon the Sorcerer II
Discworld Noir
The curse of Monkey Island
Ultima Online
Chrono Trigger
Fallout 1&2
FF VII
Silver