Personal Favorite: Chemical Plant Zone. (Sonic The Hedgehog 2)
Deep Cut: The Hyperfunk Zone from Toejam and Earl 2: Panic on Funkotron
Hard to think of levels in modern gaming these days. Lines are blurred a bit as the distinction of level is less self contained than they used to be.
Mega Man X intro stage. Masterclass in teaching you how to play the game. Great music. Memorable boss fight you can’t win. Then Zero comes and is a total badass shooting Viles arm off. Also his theme song is amazing.
I don’t know if this is an obvious pick but I think The Undead Burg in Dark Souls 1 is a stunning piece of level design that teaches you every core design principle about the game.
I have to give it to Effect and Cause (Titanfall 2).
Honorable mentions to Locomotion (Uncharted 2), Downsize Surprise (Astro Bot), Milkman Conspiracy (Psychonauts), and Deep Stone Crypt (Destiny 2).