I always liked all of Bowser's kids. It's pretty cool how they all had different personality and how different they were. Other than that, Bowser himself has to take #1 spot for me. You're only as good as your enemy, after all.
I vote for the boos, if only because there are several amusing theories and memes about who they may have been in their pre-death lives (an incredibly redundant phrase, I know): E.g. all of the boos are the ghosts of the lives that Mario lost and that's why they can't stand to look Mario in the face, King Boo is the ghost of Bowser, etc.