Depends on what you mean by best.
Kurt Cobain is one of the best songwriters ever (love or hate his music, the impact was undeniable).
Yngwie Malmsteen is probably the most technically proficient to ever exist, but his music isn't really enjoyable to listen to.
I believe the best measure is "who inspired the greatest number of people to pickup a guitar and create other popular music."
To that, I'd have to say James Hetfield of Metallica. Besides being an absolute animal in rhythm playing, his riffing style permeated across 4 decades of music in numerous genres. Some would argue Tony Iommi was more influential, but I'd disagree as his playing was much more blues influenced compared to Hetfield's diatonic riffs.