CrossCode - My favorite RPG of all time, and that's a very high bar. This is a game that is perfect in every way and adds up to more than the sum of its parts. Fantastic combat, tons of side content, endearing characters, emotionally powerful story, beautiful visuals, amazing soundtrack. I could gush for hours about Lea as a protagonist, but it's surely better to experience this one for yourself than let me get into spoilers.

Crypt of the Necrodancer - Rhythm game/roguelike sounds like the strangest mashup ever. But what's even stranger is just how well it works. It's just a matter of keeping 4/4 time, but forcing a steady pace forces you to think fast and not make any mistakes. Every death is clearly your fault as every enemy is designed to be beatable using only a base dagger without getting hit (and indeed there's a challenge character that forces exactly this), but dealing with swarms is where it gets complicated. I especially recommend trying to speedrun, playing for speed really makes this game adrenaline-fueled as you have to pace yourself judging how much time you can afford to gather items if you want to make sub-15 or sub-10. Danny Baranowsky's soundtrack absolutely delivers. Get both the Amplified and Synchrony DLCs.

Mega Knockdown - A turn-based fighting game that's fun for complete beginners to pick up and play, while still offering a lot of depth. Use this to entice your non-fighting game friends in. Also come show up to the monthly round robin league.

Petal Crash - My relationship with versus puzzle games remains complicated. But Petal Crash is still a very good game.

Slay the Spire - Roguelike deckbuilder, basic idea is that after each combat you get to add a card to your deck, plus collect relics from elites/events/shops/bosses which provide passive effects. There's a ton of depth in trying to assemble the perfect deck one card at a time, resource management gets very complicated balancing what you need in the short-term versus what you want to take to the endgame. Tons and tons of possibilities, you can pretty much expect to never build the same deck twice. Oh, and did I mention there's 20 levels of hard modes once you think you've gotten A0 down? Also has a very powerful mod API with Steam Workshop support, check out Packmaster and Adventurer for my favorite must-plays.

Ultimate Chicken Horse - Start on a nearly empty platformer map, each round everyone adds one object somewhere on the map then you all try to finish the level. Whoever finishes gets a point, plus bonus points for whoever finished first or collected coins that have been placed. Then you add another set of objects and repeat. Quickly becomes hilariously chaotic as you try to figure out how to balance screwing everyone else over while still making sure you can win, only to realize that after a few rounds you have all built a horrifying monstrosity. Has full crossplay with console versions.

Anything by Zachtronics - A bunch of different engineering puzzle games where you have to write code or build a machine to solve problems. Once you've solved the puzzle, you can see a histogram comparing your solution to everyone else's on a few different metrics, encouraging you to go back and try to optimize it further. I recommend Opus Magnum as the best entry point.