So many to list, so little time. For what it's worth, I mostly play 2D sidescrollers, which is what most of these will be:
Bomb Chicken - A puzzle platformer where your only action is laying bombs. This provides two benefits (giving you height and providing a form of offense) while also providing a drawback (nothing is stopping enemies from destroying your bomb stack prematurely). Starts out simple, but later levels can be crazily complex.
Cuphead - Besides the art, the boss battles are just the right amount of challenge. Impossible enough to make you work at defeating them, but with an ample learning curve to never make them be frustrating.
Ender Lillies - A soulslike Metroidvania without the more annoying souls elements, all with some great classical-inspired music and challenging but fair boss battles and quite a bit of customization in your choice of weapons.
A Hat in Time - An excellent 3D platformer with a variety of themes, adoringly cute, and a very satisfying jump-jump-dive-cancel moveset.
Kamiko - A very short and simple top down action game with four large maps that needs to be explored to find the four shrines in each one. Incredibly simple, but reminds me a lot of the original Zelda which is nothing but awesome in my eyes
Minit - A crude top-down adventure game with a unique twist - you automatically die every 60 seconds. That means that you KNOW the next collectible, next story beat, next spawn point, next shortcut you can open, etc is less than 60 seconds away (actually less than 30 since that is hard mode), the secret is in trying to find it.
Shantae series - Charmingly cute, often funny, and feels wonderful to control. Admittedly, the first game is a GB game and therefore not that great, and the second is a digital DS game and also somewhat limited. So just play the third (Shantae and the Pirate's Curse) and go from there.
Splasher - A platformer made by ex-Rayman devs with long, brutal levels (fortunately with generous checkpoints) that involve spraying either sticky goo that allows you to run up walls/ceilings or bouncy goo that turns the ground into a trampoline.
Steamworld Dig 2 - It doesn't do any particular thing amazingly well other than a perfectly tuned gameplay loop. You dig through the mines, eventually run low on resources and return to the surface and amazingly seem to have just enough money made from mining to purchase that new upgrade you wanted to dig further and find the cave you're looking for and switch from mining to platforming at just the right time before mining gets boring.
Sunblaze - OK, I'm actually in the middle of this one at the moment, so maybe it turns into a pumpkin later or maybe I'm still in its glow. But it's a precision platformer with some puzzle aspects, as a lot of the elements on the stage also get manipulated as you move around. 600+ deaths and counting, and I'm still going back for more.
Touhou Luna Nights - A bog-standard Metroidvania in most respects, but also with bullet hell bosses. And a unique time-stopping mechanic. And a mechanic where not stopping time but letting bullets get close to you restores your health. It makes for some excellent risk-reward scenarios.
