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.