If you think Super Meat Boy is hard oh boy do I have one for you.

The End Is Nigh is also an Edmund McMullen platformer, but with a much higher emphasis on precision. The game is technically short, but there are just so many easy ways to die that you have to get good to beat it.

It also has a little modding community that has produced some even more nightmarish levels to go along with it.

Unironically old school runescape. Playing it for years ingrained the concept of efficiency into me. Now I'm able to do well in games where mechanically I'm still shit because I'm constantly trying to use my time as efficiently as possible.

Counter Strike got me through hard times in school an taught me how to communicate efficiently. It also got me over that cringy "i rage in video games" phase that many people are still in.

Rainbow Six Siege made me much better at FPS games.

Metroid Dread. The quick thinking and reacting to avoid the E.M.M.I.s has helped me. But also raises my adrenaline whenever I hear that music…

Mass Effect made me far better at multitasking and not letting myself get tunnel vision on an objective.

Sure I'd played Gears of War, or RTS's that used the traditional rock, paper, scissors method of unit dominance, and resource management.

I'd just never played a third person shooter that expected me to combine all of those skills into a single gameplay loop which required constant shifting from power/defense based problem solving to accurate shot placement and squad positioning on the fly.

Celeste, Guacamelee, and Monster Boy in the Cursed Kingdom all made me better at platforming games.

StarCraft II made transitioning to League of Legends easy. I also played a lot of Kovaaks which made my aim generally better in FPS games and it helps with osu! too.

Mirror's Edge. I would try over and over to improve times on the time trials, and I found that I did better when I took a break and relaxed for a few minutes, or overnight, before trying again. It has improved my playing other games as well as my real life efforts

The last of us multiplayer made me a better sniper. Smash bros amplified my reaction time. And halo improved my hand eye coordination with grenades.

Bayonetta, and Burnout 3. I got really good reflexes and timing.

And way way back when I was super absorbed playing Manhunt I got uncannily good at spotting dark shadows at night to hide in.

It's not really in the spirit of the prompt, but learning the NMG speedrun of LttP has really improved my movement efficiency in games simply because I'm always thinking about it now

Dead by Daylight. Spending so much time chasing or being chased by other players has definitely improved my tracking/flanking abilities in other games, as well as pathing and following players audibly. I may not be a great shot, but I know exactly where you’re going and how to cut you off.

Also utilizing mindgame strategies can really fuck with people in other games.

Edit: Also also, I don’t get tilted nearly as easily as I used to. DBD sucked all the rage out of me.

Edit 2: My first point also applies defensively, in that I know how to more effectively lose someone if necessary.

I feel most improvement playing turn-based games, like TFT, Hearthstone, Slay the Spire.

Counter strike makes any single player shooter trivial. The best they can do for difficulty is make headshots not actually kill the target in 2 or 3 shots.

Platformers are easy after super meat boy.

The original Deus Ex will train you to explore and scrounge for every bit of ammo. Making your skills from counter-strike even more valuable.

Nerf Arena Blast. The first FPS I ever played. Got so good and bunny hopping around.

The speed and random nature of Spelunky really helps build gaming skills.

DayZ SA hardcore, my reactions have improved and I have become more cautious as a player, especially of people.

The original Driver on PS1 made me better at handling cars in general, both in video games and real life. It's a shame GTA and Saints Row went with exaggerated vehicle physics and the Driver series never got the non-vehicular parts right.