Since a lot of the ones mentioned so far seem to be shooter levels (and I agree fully with Effect and Cause), let me throw in some platformer ones. (And I have a few, so sorry, this might get long).
It’s been a while since I’ve played Dead Cells and they might have changed it a bit in an update, but once I got decent enough at the game not to die instantly the moment I entered, I started really loving Fog Fjord. It feels like its enemies are balanced perfectly between needing to plan well and then execute well, especially between the boomerang-axe ones and the cannonball pirate ones. And that music is dope.
Chapter 6 (Reflection) in Celeste is a tightly-focused yet still expansive feeling level that treads the line between precision and some really satisfying kinetic platforming. I think my mouth was open for most of its boss fight. And even though I’ve heard a lot of dislike thrown around, I really love its bumpers.
City of Tears from Hollow Knight is the epitome of everything that game does well: a combination of intricately linked spaces and hidden combat arenas inside an incredibly atmospheric and moving setting that manages to tell a vast amount of story with almost no text.
Some honorable mentions: Hydrocity Zone from Sonic Mania. Flying Machine from Shovel Knight (which imo might have the best level design of any platformer). Also, going back to shooters, I really love the Half-Life 2 vehicle levels, but I recognize that I am fully alone on that one. Still, Water Hazard for life.
But no, the greatest video game level ever, without question, is Mariachi Madness from Rayman Legends. Also the rest of the music levels, but that one is just… listen, if running through a perfectly engineered speed machine to an acoustic version of Eye of the Tiger while colored skeletons play guitars and kazoos is not your idea of a good time, then you’re just no fun. (I’m joking about that last bit, but seriously though, it’s great.)