So, most of my recommendations are going to be FPS or first person. For Valve related stuff:

• Half-Life 2 is pretty much a must have along with Episodes 1 and 2.
• Black Mesa is a remake of the original.
• Entropy : Zero 2 is a fantastic fan made mod that’s a good follow up after finishing Half-Life 2 and the episodes.
• The original Entropy : Zero is pretty good, but the default difficulty is hard as hell.
• Portal 1 and 2 are also a must have
• Portal: Revolution is an independent mod that’s a good follow up after 2.

As for non-Valve games and related:

• Crab Champions - fast paced, fps, rogue-like shooter.
• Talos Principle 1 and 2 - fps puzzles with a serious philosophical scifi story. It actually gets a bit heavy when it discusses death.
• Untitled Goose Game - Honk!
• Gnorp Apologue - fun little game
• Pineapple on pizza - it’s free. I would describe it as games-as-art.
• Any of the Serious Sam games. They were made for PC.
• Fallen Aces is still in early access, but a good story driven retro FPS with sprites and multiple ways of navigating each level.
• Trepang2 - a bit of an odd FPS with bullet time.
• Roboquest - rogue like FPS
• Exit 8 - horror walking simulator. Kinda short once you figure it out.
• APE OUT - Ape smash! Top down “shooter”.
• Hotline Miami 1 and 2 - top down shooters with fast deaths and excellent soundtracks
• The Binding of Isaac - has always been popular if a little dated
• Gunpoint - stealth puzzle third person. Came out a decade ago, but a good game.
• Party Hard - little indie murder-everyone-and-don’t-get-caught.
• The Stanley Parable - walking simulator with some interesting dialog and interactions. 9-to-5 office people can relate.
• Dusk - awesome boomer shooter
• Antichamber - came out a decade ago, but a it’s a mind bending puzzle fps.
• Hades and Hades 2 which is in early access. Third person isometric rogue-like.
• The Invincible - Story driven walking simulator based off the book of the same name. Good story.
• Firewatch - story driven walking simulator
• Deeprock Galactic - wasn’t my pint of beer, but a lot of people like it.