Best: Rosalina, because I'm a Rosalina fanboy and no one has brought her up yet, though she's only been playable in one game so far. Second choice would be Dry Bones, who has become my favorite playable grunt.
Worst: Petey Piranha. Waluigi and his fanbase are annoying to encounter, but at least he's not a worthless boss who for some unknown reason got handpicked to appear in every Mario spin-off since his debut. Thankfully he's absent from Mario Kart Wii.