A tough question because I like almost all Wario Land 4 levels, but I think Mystic lake, with Crescent Moon Village being a close second.
My least favourite level isn't a hard question on the other hand: that's Fiery Cavern, which is also the level with my lowest hi-score.
I like Golden Passage and Domino Row; they're both tricky but also super satisfying when you get stuff right. Monsoon Jungle is also fun and brings back a lot of good memories of me playing the game badly when I was younger.
I see Monsoon Jungle is very well-liked, and I love the level too, especially the feature that you can go from deep underground to literally through the roof, and the long secret cave in the post-switch part.