Those who know me a bit can probably guess my answer beforehand...
Best: Count Bleck. I'll avoid mentioning the secret backstory, for those who haven't beaten Super Paper Mario, but it's the reason he's so unique and awesome as a villain. His mannerisms and personality are great too - "What!? spat Count Bleck in utter disbelief." XD
Worst: I'm tempted to go with Bowser, just because of his extreme overusage, but he's been developed as a character, and sometimes he's good for comic relief or actual awesome villainry. I'd have to go with...Wart. There's nothing that makes him stand out at all. An evil toad who shoots bubbles trying to take over a dream world...yeah, real creative and original. Of course, he was just ported from Doki Doki Panic...