I think Guy-Manuel de Homem-Christo and Thomas Bangalter are definitely musical geniusses. They've inspired so many and their influence can still be seen today, the whole pop and dance landscape would be different without them.
EDIT: Oh, you want me to define, alright.
A musical genius is someone who looks beyond what is already there, is not afraid for the unknown and is willing to take the backlash for whatever he or she stands for.