Stevie Ray Vaughan.
Sure, there are plenty of musicians who had a bigger impact on music as a whole, but NOBODY I've ever seen was more in tune with the universe than Stevie when he was playing.
As far as I can tell, he didn't actually play the guitar. He just acted as a conduit to channel music directly from the universe through a guitar. I don't think he ever once had to pause and consider what to play next when he was improvising, it just flowed out of him non-stop.
He had already started getting even better, having finally gotten sober, and it kills my soul every time I think about what else he could've given us had he not gotten on that helicopter.