A bunch of my favorites have already been listed, so I'll just mention the one that wasn't: Antichamber, a first person puzzle game that's probably somewhat like Portal in terms of how it requires you to rethink your assumptions about how space works, but it's a very different game, both mechanically and in tone. I don't want to give away too much, but it's a mix of weirdly unsettling elements (although it is by no means a horror game), a design that's actively trolling you in ways that will make you laugh, and mechanics revelations that will have you scream "Wait, I could have done that this whole time?!" It's one of those games that I wish I could delete from my memory and play for the first time again.

Diaries of a Spaceport Janitor is one of my favorite indie games of all time. The city design really captures the feeling of wandering around an unfamiliar, large, bustling place. The diary mechanic at the end of the day is a great way to get in character, and I like that you can decorate the apartment. I did some light data-mining (mostly item info and dialogue strings), and I even have fridge magnets of some of the pixel art!

Depanneur Nocturne is also a great evening’s worth of exploration and vibes, but I mention it because it has a reference to Spaceport Janitor and it made me SO happy when I realized that. :)

I'd personally recommend Rain World. It has a reputation for being pretty hard, and you'll probably die a lot so you have to approach this game with patience