Fiery Cavern by a long shot. It has the most sophisticated form of folded level design in the whole game (new enemies, surfaces, visual theme, remixed level layout, new rooms). The Iwaos (dudes with the boulders) have more back and forth interaction with Wario, so you really feel like you have an enemy that you can properly spar against. I love how for the first half of the level the Iwaos are positioned so as to give them increasing spatial advantage over Wario. It keeps the gameplay tense and engaging.
The challenges in Hotel Horror are mostly base level. There's no unifying idea behind them and they don't develop throughout the entire level. It is nice how they offer a different kind of challenge, deducing the path ahead, though.
Crescent Moon Village. The atmosphere and music just make that level, and I'd say the theme song in particular is better than near enough any other ghost level theme I can remember.