Although it's mad close between Clapton and Hendrix, I'd give it to Jimi purely on the basis that "he pushed the envelope".
And Mili, on your Pac vs. Big example...Big was never the lesser rapper compared to Pac, man. I'm kinda amazed you are arguing this. Sure, one can like Pac more, but they were two totally different people. Where Pac was the emotional, passionate, socially conscious rapper, Big was the technical genius. I mean, purely as a rapper, Big ripped Pac three new assholes. As an overall artist however, Pac trumped Biggie because of his subjects and passionate delivery. But to say, on a technical level, Pac was the better rapper compared to Big, is pure lunacy in my eyes.