Daisy with the Mach Bike, although her "wo-ho-ho" yell can be rather annoying. I try to ignore that because she's fast.

If I wanted to show off online, I'd say Rosalina just because so many of the noobs are desperate to unlock her (and I do not have a save file for whatever that other game is; I unlocked her by getting a star ranking on every grand prix), but she's not really fast enough to suit how I like to play the game.

I almost always prefer bikes to karts unless they're running a kart-only tournament.