OK, Tony McMannus is an astounding guitarist. He remains my favorite, based on the couple of recordings that I have. But I still have trouble with the "World's greatest". I enjoy Ian Carr's work with Shannon Tweed, even if he uses a--dare I say--pick. I admire Dave MacIssacs for being audacious enough to play traditional music on an electric guitar--knowing how my own efforts at playing for the Morris has raised a few eyebrows. Heck, I've been known to delight in both Julian Bream's and Pat Metheny's playing.
If you could rephrase it to "World's best FINGERSTYLE guitarist, then I would have no choice to agree, though I've yet to see him in person (do I sound jealous?).