Merle Travis
The First Modern
Of course, everyone can pick - no joke intended - their favorite guitarist. But as far as picking the most influential guitarist, if someone as innovative and even radical as Arthel Lane "Doc" Watson says Merle Travis was a major influence and that it took him about ten years to get all of Merle's notes in, then we have to hand the crown of the innovators to Merle.
Unfortunately there aren't many major academic and university biographies about Merle (in fact there don't seem to be any). Fortunately, there are other resources where you might learn a bit about Merle Robert Travis and what is called Travis Picking. But if digging those sources out - as they say in Kentucky - c'nest pas la peine, you can just click here.