Asklemmy

Rainbow Six Siege made me much better at FPS games.

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.

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

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…

Well, I can tell you this. I grew up playing Mega Man. People say those games are hard, but I have them all memorized, so they're all pretty easy for me.

Sometimes I play platformers that people consider hard, and I'm just disappointed by how mind-numbingly easy they are. Celeste is one example. I kept thinking, surely it must get harder. Maybe when I do the B sides. Surely there must be at least one part I struggle on. There never was. I never found anything hard about the game. The story was amazing, though.

So anyway, my answer is Mega Man. Not Mega Man X. Those games are amazing - quite possibly my favorite platformers of all time - but they're too easy to fit into this category. The classic, 8-bit mega Man games from the NES (Mega Man 2 excluded. That one is also too easy).

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.

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

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

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.

Nioh 2

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

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.

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.

Life

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.

Unblock Me taught me that even if you don't see the solution yet, moving the pieces in the way that they can move will often illustrate the correct path.

Apex Legends really honed my skills with shooters on keyboard and mouse. I always thought I was terrible at shooters. Turns out it was just the controller I was bad with. I always played with friends and it greatly helped our tactical communication and snap decision making skills too. We found out who the natural shot callers were and who can't be trusted to make the calls in a tense situation, lol

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.