King Boo, pretty much the only Boo I actually like. I like how he's the kind of villain who actually takes care of his minions and actually sees them as friends not just slaves who have to do whatever he says. Although I would like some clarity as to whether or not he's allied with Bowser. Sometimes he works for Bowser, other times he sides with Mario and Co. (Mario Super Sluggers) and sometimes he's neutral (Mario Party 8 ) .
Bowser's just awesome. Take two awesome reptiles (turtles + dragons) and this is what you get. Honestly, he could've ruled the whole world if the good guys and new bad guys weren't in his way, seriously, he has 8 spoiled brats to carry on his legacy, an elderly master of dark magic who raised him, an army of turtles, angry mushrooms, ghosts, walking bombs, and vicious flowers at his disposal each with a subordinate king. Plus, Bowser's one of the few villians I can think of ATM who actually defeated his arch-nemesis and left him near death. On top of all that, Bowser's the "Even Evil Has Standards" type of villain, my favorite.