Mine is certainly Count Bleck from Super Paper Mario - he's a very interesting villain just in terms of appearance and mannerisms, but what really made him the best Mario villain in my eyes was that he had a real backstory and a motive for what he was trying to do instead of "generic evil dude wants to rule the world", and it was pulled off really well, especially for the first time in the series dealing with a villain who had a complex backstory like that.
My second favorite, also from Super Paper Mario, is
Dimentio, for many of the same reasons. His backstory wasn't developed nearly as much as Bleck's, but he had quite an amazing and intricate plan and it was very exciting to see how it concluded at the game's climax.
...But since neither of them are on the list, I'll have to go with my third favorite, Fawful. He's just so wacky and his butchering of the English language and use of weird and extended metaphors are always hilarious. "A WINNER IS YOU!" "Your lives that I spit on are now but a caricature of a cartoon drawn by a kid who is stupid!"
Darth Butch.
Ahahaha, I remember that guy. :P