Wario Land 4's levels are honestly some of the best I've ever seen in platformers, taking some of the aspects from Sonic games about learning the levels in order to improve your play and having loads of nooks and crannies to promote exploration.
So I can't really pin anything down, but I feel Crescent Moon Village does this the best.
It offers alternate routes, skill factor, loads of secrets, and a really cool unique gimmick.
Speedruns of the level are bangin' the player never has to stop for the majority of the level.
Also the music is great.
I remember having the most fun playing 40 Below Fridge, and since all the aesthetics and music are all really fuckin' great for such a short game, 40BFridge is for me what CMV is for CM3...0. in my experience there were a lot of levels that felt kind of clunky. By far the worst part about CMV was those goddamn mailboxes just sticking up out of the cobble. There are plenty of things about CMV that I do like, and most of them are based in aesthetic. I do still like the stage, and the thing that Durdge said has made me think about it in a new light (and indeed pretty much all of WL4's levels), as a whole I feel like if they'd made it a level from later on in the passage maybe it wouldn't feel like (somewhat) wasted potential.
Crescent Moon Village for me as well, though I always had a strong affinity for Hotel Horror, 40 Below Fridge and The Big Board as well.
Land 4 in general stands out for how much of it's own identity every level has, in theme, visuals, audio and how they play, makes it hard for me to rank them outside of a few select favorites, because even Hall of Hieroglyphs, despite just being the tutorial level, has a lot of charm of it's own.