Syrup, Cannoli, Carpaccio, and the Shake King are motivated by stealing to gain wealth. In that respect, they're really just about as evil as Wario himself is.
I dunno what Demon Head's (referred to as Awazon Genie here) deal was. He was just VBWL's final boss and had little story explanation.
Rudy enslaved the Music Box world and turned its inhabitants into monsters, which is a higher grade of evil. However, him suffering from a cold in Dr. Mario 64 did expose a slightly more comical side to the character.
Golden Diva seems similar to Rudy, transforming Shokora into a cat and taking over her lost kingdom.
Black Jewel seemed to just pick on greedy people specifically. He seemed to be some kind of karmic trickster who transformed Wario's castle just to punish his greediness. Of course, Wario was having none of that.
Terrormisu straight-up destroyed an entire ancient society and convinced a pharaoh to starve his subjects. She's by far the most maliciously cruel of Wario's foes, so she gets my vote here.
Of course, "most evil" does not equal "favorite." Those honors go to Syrup and Cannoli; Wario just works best off of rival thieves.