Asklemmy

A couple of oldies, that deserve to still be played. Disclaimer: I played both games when they were already ~8 years old, and completely outdated in terms of technology.

Planescape: Torment

One of the best RPG ever created, and that is entirely for the world building and writing, and how much of the gameplay ends up being based on these rather than the combat mechanics (which are just ok)

Deus Ex

Again it was way ahead of its time in terms of world building and depth, and it was still an unashamed PC game, that dared to challenge its users a little and didn't need to have a GUI that could be used with a gamepad, unlike the sequels.

Kerbal Space Program

Civilization III and/or V

Edit: If you have lot's of time available.

Terraria

Celeste absolutely! It's difficult but it's really really fun and has a great story. If you ever get super invested, the community is great and the skill ceiling is so high that you can always get better when playing new maps.

Hollowknight my beloved

I’ll do you one better: completely free.

Check out Ashes: 2063. It started life as a mod for Doom, but is now completely standalone and has more in common with the Metro games than anything else at this point. PC only, but both games and their expansions are 100% free and worth every minute of your time.

There was an outstanding RTS game from 1997 called Total Annihilation. The soundtrack was 11/10, it was made by Jeremy Soule, the guy who did the Skyrim soundtrack. Some might say that Supreme Commander 2 is their most recent successor to TA, but they are wrong. Check out Beyond All Reason (BAR). It's in Alpha and it's free but it's already sooooo good.

Valheim

Dead Cells. Factorio. Mini Metro. FTL.

Katamari! It's just such a wild and fun experience. Even the sequels.

Nier Automata: It's a game that uses every aspect of the medium. From the story, the music, game play, even the end credits, it feels like they took such care in crafting a memorable experience and didn't let anything they could use to express themselves go to waste.

What Remains of Edith Finch, specifically Lewis' chapter. I think it's an excellent portrayal of how incedious and overwhelming depression can be.

Telltale's the Walking Dead (first one only): I was not prepared for the story. I wish they took the same care in the sequals, but I feel like the quality of the first one just came out of nowhere.

Any Monster Hunter. I just think it's neat.

Mount and Blade Warband for me. Native is fun but the community has made so many total conversion mods for it that can be swapped in and out at the launcher. It can be Game of Thrones, Lord of the Rings, most points of history between now and ancient Rome, and it can even be Star Wars

Chess

I only gotta mention one that everyone should play at least once in their lifetime: Legend of Zelda: Breath of the Wild