Deus Ex: Played it more than once, very very good for its time! Rough graphics for today, even modded, but also really cheap for the game quality. If somebody really loved Prey or Dishonored, I would point them at Deus Ex for sure.
Diablo 2: Played it, but not for long. I bought the whole 'war chest' version, too. Arg. I just don't like the Diablo-style of gameplay. (P.S. The way your character chugs down potions in that game, you would think that bathroom breaks would be a central mechanic!)
Doom: Oh hell yeah! I bought the whole thing, played through more than once, made my own WAD, and so on. Now it's best played as a freebee inside one of the last two Doom games.
Grim Fandango: never played it. Something else cool must have showed up at around the same time?
Half Life 2: Fun game, but I only played through once. I'm still waiting for Valve to finish all the episodes and sell them as a set. (eye roll)
Minecraft: Never played it.
Myst: Pretty good stuff, but I wouldn't say I loved it. I think I managed to get through it without help? Or maybe help with one specific puzzle? Whatever the case, I think I heard Richard Corbette blow some chunks when I read it.
Planescape: Torment: A really good one with excellent characters. I only played through once, but I enjoyed it a lot.
Quake: Played it all the way through the single player campaign, but it was pretty meh. The soul of that game was the online play, and I didn't really want to do that.
The Sims: Haven't played it.
My 10ish
City of Heroes
X3 and X4 from Egosoft
Witcher 3
Final Fantasy 7 (original and Remake)
The Last Remnant
Skyrim
Troubleshooter: Abandoned Children
Xcom2
BattleTech
Outer Wilds
Saints Row: The Third
Yakuza 0
Subnautica
No Man's Sky
Doom
Deus Ex
Baldur's Gate 2
Batman City
Portal 2
Ultima 2
Descent
Star Wars: Rebellion
BattleZone
Sacrifice
Just Cause 2 and 3
Shadow Hearts: Covenant (console only)
Atari Adventure (console only, at least for me)
Empire (played on a PDP10 mainframe)
NetHack
War in Russia
Oblivion
Railroad Tycoon 2
Duke Nuke'm
Vampire: The Masquerade: Bloodlines
Mechwarrior 5
Civlization 5
Am I up to 10 yet??
A lot of those are games that I loved at the time. As GOG has shown me time and time again, several of those old ones aren't
nearly as fun now as they were when I played them.