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

DOOM 2016 on Nightmare (not ultra)

Sniper Elite

Hotline Miami

Dead Cells

Dying Light on Nightmare

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.

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.

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.

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

Zelda 2 Adventure of Link. I didn't have Castlevania or Ninja Gaiden as a kid so that was my Nintendo Hard game lol.

TF1, Q3 arena - at least for multiplayer.