In my view, if you play ten intruments well, you're not as talented as someone who can only sing, but sing amazingly.
Nevertheless, I'm with AG on Prince v. MJ. Michael was a reasonably good singer in his day, and he was an excellent dancer and performer. Prince was a reasonably good singer, great dancer and great performer, plus a masterful composer and yes, he played multiple instruments amazingly. I don't think there's any question that Prince is a musical genius. To me, the bottom line is that musical genius beats "King of Pop" any day, all day.
Stevie vs. Prince: I hate to do it, but I have to go with Stevie. Same reasons given by AudioGuy.