Although Clapton was / is a big fan of Prince, the "dont know, ask Prince", is an urban myth. First appeared when Hendrix was reported to have said the same thing about Rory Gallagher.
There is no best guitarist ever, although Clapton is my favourite, because when he is in the mood and plays like his life depends on it, there is nothing that sounds better to me.
The best technically Ive seen, up close and personal, was Gary Moore. He could play anything, backwards, forwards and twice as fast as anyone else if he felt like it.
Other Favourites - Stevie Ray Vaughan, because he almost always played every note as though he might never get the chance to play it again. Rory Gallagher, for his authenticity, and always remaining true to himself, despite the personal cost. He was by far the greatest live performer Ive ever seen. As well as being a lovely shy unassuming bloke, off stage.
Jim Armstrong. The best guitarist no-one has ever heard of.
Prince was a very under rated guitarist, and artist. He could play the guitar like Hendrix, while dancing like James Brown.
Not an easy trick to pull off.
Swmbo is a huge fan if Prince. She has played me some vidoes of him playing guitar live which are pretty incredible.
I wasnt overly impressed with the "while my guitar" performance personally though.