Noita is a 2d sandbox roguelike built on an engine inspired by old "falling sand" flash games. It's extremely deep, contains tons of secrets that are extremely obstuse, and features a wandcrafting mechanic that is designed to be broken with mathmatics and logic. It's mean, so use the steam workshop to soften the difficulty if you need to (I did).
Hotline Miami is a game that forever changed my taste in games and music, but Katana Zero is such a tightly woven piece of art despite it's erratic development that it beats the source of it's inspiration for my recommendation. A beautifully told story with sharp gameplay and a pretty killer soundtrack.
I played it on the pcsx2 emulator during a time where I was playing a lot of classics to see if they hold up. You will fight some jank in the controls and combat, but it's beyond worth it to push through this game. Avoid anything online about it unless you need to check a walkthrough.
A bunch of college students turned a technical planetary simulation into a full game, and packed it with so much excellent exploration, sights, and atmosphere they made something immensely special. Worth your time slowly working through it. If you have somebody to sit on the couch next to you a couple of days a week, do that.
A classic FPS game that has more convincing AI enemies than anything I've played since. Halo and Half Life broke ground with their enemy variety, but the replica soldiers you are fighting 90% of the time here have so much unpredicability and tactical counterplay to them that they hold up through it's entire runtime. Trepang2 is a good spiritual successor, but doesn't beat the classic.
Tarkov is a soviet PvP game that does better long term progression and hardcore gunplay than nearly anything else. Being good at Tarkov will require you to learn a lot about how guns and the technology surrounding them actually function, and it is absolutely NOT worth putting up with the terrible servers, matchmaking, cheaters, and enemy AI to get all of that. So, we mod it to be singleplayer with SPT. Then we mod it further to improve the balance, bots, gunplay, and new player experience and add back in self hosted co-op with friends using Fika. Worth the setup even if you can't bring friends, but if you have other gun nerd friends... Few games compare.