Mine is Waluigi, partially because I hate life and partially because no one else picks him. Also that annoying voice brings a smile to my face.
I usually go with either my Mii (it's five o'clock shadow and constant frown looks so out of place it makes me laugh) or shy guy. Shame they wasted so many spots with babies, metal variants and Koopalings. Hope the next game is just a Smash Bros. type deal with characters and tracks from every franchise.