Celeste
Probably my favourite game if I was forced to pick. Discovered it in a weird time in my life, and with the message of the game and the really special soundtrack, it ended up having a massive impact on me. I can't really divorce the game itself and that part of me, so it ended up kind of being a formative piece of media in my life and it will continue to mean a lot to me as long as I continue to struggle with anxiety. Anyway, enough about me.
The game's soundtracks are, imo, my favourite in any game I've played, and you should definitely give them a listen, or better, play the game, because they really blend with the environment and gameplay like no other game I've played. You can find all three albums (A-side, B-side, and Farewell) on Lena Raine's bandcamp page.
The gameplay is practically perfect. Madeline feels great to control, and is very responsive. Out of all the platformers I've played, Celeste feels the best. It really feels like Madeline is an extension of me, and controls naturally. I never felt like I wanted to do something, and it didn't work due to the game's controls or physics. They really nailed the core mechanics of the game and so it feels amazing to play.
The design of the game itself is also really great. I never really felt genuinely frustrated while playing Celeste, which I wasn't expecting when picking it up (because I'll be the first to admit I'm not very good at games). Celeste does a lot to set expectations when getting into it that you will die a lot, and makes dying and trying again as smooth of a process as possible. When you fail a screen, you immediately pop right back in. No dramas, and no consequences other than an opportunity to try again. Celeste is also really good at slowly introducing concepts, and ramping up difficulty over time so that you aren't suddenly hit with a super difficult challenge that you don't feel like you don't have the skills to overcome. Over playing the game, I really always felt that there was a way forward, so I didn't really feel unmotivated to face the next challenge, or felt lost. So I was able to complete the A, B, C, and eventually Farewell (a massive jump in difficulty) while being guided and slowly introduced to the required skills over natural gameplay (barring the cute slideshow in Farewell). I did this with probably more deaths than the average player, but had a lot of fun the entire time!
Mods
I just started modding celeste and I've been having a lot of fun playing the Strawberry Jam Collab. It's a massive mod that adds 5 massive lobbies with 111 new maps in total (essentially self-contained chapters), with each chapter bringing their unique spin or mechanic to the game. It also has its own original soundtrack by many different contributing artists from the community available on bandcamp. Each lobby gets more difficult (from beginner all the way to grandmaster), with each lobby teaching you the required techniques to complete it in a special 'gym' stage. Everything is really nicely explained, so if anything this acts as a great teaching resource! Don't feel intimidated to try it out as a lot of the levels are around A-side difficulty. As mods go, this really has to be as good as they come in quality and care.
The Foxeline skin is pretty cute.