Favorite criminal: Vermouth. Unpredictable, smart... she is the one factor that keeps changing, that you are never sure of.
Greatest criminal: Gin. Cold, ruthless. And smart. The stereotyped bad guy in balck trench coat with a black hat, the icy eyes... but so much more than a stereotype. Professional from the beginning.
I love the BO. They make the story so much more interesting by being the shadow of a threat, the threat of a shadow, all the time, never to forget and yet so easily pushed into a far corner of the mind ^^
I would have voted Kaitou Kid but I don't think of him as a criminal (ok, he is International Criminal n° 1412, but he just is NOT a real, hardcore criminal).