Uh… I swear I wanted to contribute just 2 or 3 games, but as I wrote, I kept remembering one gem after another… oh well… :)
Outer Wilds - So hard to describe, it’s an exploration game, but what you’re exploring is a star system going supernova, in a wooden spaceship no less. And a strange way of (not) time travel is also involved, which could be the root of the whole game loop.
Axiom Verge - A platformer that is such a labor of love that it hits just the perfect mix of approachability, exploration, story development and that “huh?” factor where right until the end you’re not sure what your abilities actually mean - i.e. if you could glitch through walls in the real world, would that imply the real world is a simulation?
Stardew Valley - A somehow utterly satisfying farming simulator in the style of the first Harvest Moon games. Such a nice getaway game - it begins with your avatar quitting their office job and moving to a farm inherited from their grandfather. No taxes, no boss, no stress, just rise with the sun, plant, water, harvest and fix. Change your rhythm with the weather and the seasons, investigate charming little mysteries of a beautiful place.
Broforce - Another platformer, this one a bit more brutal. Far over the top 80s action heroes bring freedom to the world, but whether you play as Robocop, Schwarzenegger, McGyver, Snake Plissken, Ripley or another 50 heroes is almost random and each hero has completely different weapons and skills. Destructible environment and even a large Xenomorph outbreak (how the heck did they get the license or grant?).
Protolife - This one uses such a madly simple recipe for complex gameplay. Seen top-down, you’re a robotic loader than can put down dots. That’s all. But certain arrangements of dots are guns, long range guns, flame throwers, area denial, missile silos, barriers and so on. You’re attacked by insect-like creatures, but instead of building tanks, you have to attack via well-placed guns slowly pushing the swarming enemies back.
Alien Shooter 2 Reloaded - Simple top-down shooter where you’re the lone soldier seeking to contain an alien outbreak. Goes for the time-honed recipe of character stat upgrades (speed, health, accuracy) and purchasing weapons and weapon upgrades. The interesting part is the insane hordes you’re up against and that all the corpses stay. It’s not unusual for entire corridors to turn into flesh hallways of blood and carapaces.
Moons of Madness - I hope this is actually indie, the graphics are near AAA level. It’s 50% walking simulator, 50% cosmic horror, set on Mars. You’re an astronaut doing maintenance on an outpost, but rather than go for the “freaky alien attack” recipe, reality itself seems to be somehow bending. Cthulhu, is that you?
Lumencraft - Top-down game. You begin as a miner in an underground base. Something really bad happened to humanity and now you’re digging underground for metal and for “lumen.” To feed the reactor that keeps humanity alive, you have to meet harvesting goals and dig tunnels, but various enemies attack in waves, so you have to spend part of your resources on fortifications and turrets and avoid opening up too many avenues into your bases.
Carrion - 2D platformer-ish. In a secret place, scientists are holding a horrific, tentacled bioweapon locked away, but it escapes. Twist: you are the tentacled bioweapon, slithering through pipes, circumventing security systems and trying to escape from the lab.
Nuclear Blaze - 2D platformer. You’re a fireman sent to contain a fire the broke out in some kind of installation in a forest. But one building has a shaft that leads deep underground where a high-end containment facility is suffering a failure. Takes place in the “SCP” universe and your only tool is a fire hose. Extremely fun trying to extinguish fires in a way where they won’t spread again.
Mothergunship - This is a first-person shooter where you’re bording and destroying (from the inside out) an army of AI space ships. But instead of a traditional gun, you have gun parts you can stick together. How about a triple rocket launcher with two shotguns in the middle? Or a shield generating laser with a sawblade attache to it, and maybe two shotguns just to be sure? It doesn’t grow old with new weapon parts being introduced right until the very end.
Space Run - 2D base building. You’re a mercenary cargo pilot fending off space pirates. But you don’t do it by controlling a turret, instead, your spaceship is a building surface and you have to build the right kind of engines, turrets, shields and power generators (in mid-flight no less) to be able to shoot down incoming rocks and pirate ships. Extremely well balanced and fun.
Creeper World - 3D real-time strategy. But your enemy is not actually present on the map, you’re just fighting a simulation of liquid, a gooey slime that pours out of several spots. You have to keep shooting, bombarding and containing the splashing, pouring slime until you can neutralize the slime outlets. The story is cool, too. The slime is actually some extinct species “gift” to the universe which dissolves everything into data, transmitted to some eternal storage space at the center of the universe.