Xbox Series X/S.
It isn't even particularly bad by itself, but compared to its predesessors (Xbox One and Xbox 360) the Xbox Series X/S gamepad is a clear step back when it comest to build quality (just try pressing the D-Pad buttons without thinking "this is cheaply made"), and that comparison is what makes me hate it.
And what adds insult to injury is that the quite expensive Elite version of the controller is just as cheaply built as the regular model...
Technically I'm still playing "Vagrus - The Riven Realms", but I didn't play much lately, since I rediscovered my love for the Lean4 programming language and am now playing around with a formally validated heap again.
There is a Wikipedia article about this: https://en.wikipedia.org/wiki/List_of_open-source_video_games
It is, however, vastly incomplete, as entries without "reliable sources" get deleted. Mind that linking the source code repository, the steam page, the license file and news about a game going open source are not enough to count as "reliable source".
I'll go with "less than 100 reviews", as with "less than 1000" my list would get really, really long. This leaves two titles in my Steam Library which I think deserve way more attention than they got:
Not really that big of a deal, but Baldur's Gate 3 can be launched with the --skip-launcher command line parameter to, well, skip the launcher.
I wanted to be a bit more productive in my spare time, but I have made a huge mistake:
I started playing Vagrus - The Riven Realms again.
The world building in Vagrus is excellent. It's set in a post-apocalyptic fantasy Roman empire, and there is a massive amount of text that details the world, and the people living in it. There is so much to read, that the devs even thought it necessary (and rightfully so) to display a warning about the sheer amount of text on the game's startup screen, with the suggestion to refund it if one doesn't enjoy a lot of reading. Sooo, of course this is the perfect game for me - or would be if I had more spare time.
The game is a mixture of trading sim and role playing game. You play a vagrus (a caravan leader), and travel the land trading wares, transporting passengers, spreading gossip and doing missions for different factions, you also have a lot of story elements that you can (and should) follow. There is turn-based combat, and during story events there are plenty of skill checks.
The game is relatively difficult, due to its interwoven mechanics. You need to calculate relatively tightly in order to make a profit, but if you loose people in combat, not having reserves might lead into a morale-loss and hunger death spiral... Also, due to the game's grim settings, the choices one faces are more often than not to either do what is right, or to survive.
My top answers are of course Kerbal Space Program, Dwarf Fortress and Stellaris.
However, all those have been mentioned already, so, to add something new to the list: Pathfinder: Kingmaker. It is currently my favourite cRPG.
Edit: Since you mentioned "Great Linux ports": Kingmaker has a game-breaking bug in the Linux version regarding Gamepad input. However, as long as you play it with mouse and keyboard (as the gods intended - insert PC Master Race meme), the Linux version is working perfectly fine. However, if you plan on playing it on the Steam Deck, you might want to play the Windows build.
And their gamepads for Xbox were pretty good too. Past tense, because those of the Xbox Series X suck (including the "Elite").
This. I had written a similar last paragraph in my answer below, but decided to delete it before submitting.
I have to suffer Windows at work. No way on earth this sad excuse for an operating system gets anywhere near my gaming PC. I want my gaming PC to be for fun stuff, not use it to torture myself.
First things first: This hasn't happened to me in ages. I even stopped looking at ProtonDB. Stuff just runs.
However, if a game I buy really wouldn't run on Linux, I would just refund it (if possible) and play something else. I have a pile of shame that could fill a hundred lifetimes, I really don't need to play this one particular game.
I'm not sure what problem you are encountering exactly, but switching to another virtual terminal might still work. By default, the virtual terminals are linked to the F1-F8 keys, and the combo to switch from a graphical session usually is CTRL+ALT+Fx.
On that other virtual terminal, you might be able to kill KWin.
Yep. With Kingmaker it was extra annoying, because the game has Steam Deck Verified rating, and the Steam Deck defaults to the Linux build.
Thing is, you can play through the whole first chapter of the game with a gamepad without issues... However, once you unlock the Kingdom Management screen, you run into the bug, which is a soft-lock once you open said screen. The UI doesn't properly initialize, all text fields remain at their default value, and you cannot make any inputs any more. Luckily the ESC-Menu still works, so you can save your progress...
If it weren't for the Steam Deck, I guess very few people would have run into this bug, if any at all. Kingmaker has a different UI if played with a gamepad, and even though Kingmaker's gamepad-UI is done really well, it is clear that it is meant for playing the game on a TV screen (think: consoles). If you are sitting right in front of your screen (PC), the UI you see when playing with mouse and keyboard is superior in each and every aspect.