Almost all of the enemies from Wario World are my favorites. Though out of all of them, my favorites are the mirror wizards from Mirror Mansion. I've always been a fan of that kind of mask, and the purple color scheme is great too.
Outside of...
... I am the singular vote for the Black Jewel.
Honestly if anyone returned it'd be neat, just to see their Mario Wiki article listing "latest appearance" as something other than "their debut game" or "Smash Ultimate Spirit cameo"
I said it in the replies to the post but genuinely Render96 could have a field day if they tackled Wario World. I always wondered about the background for Greenhorn Forest, for example.