In no particular order... and this list could be so much bigger.
Doc Watson... he passed away today at the age of 89 and was an extraordinary flat picker who adopted the style of Merle Travis. Modern speed metal freaks could take a lesson from Doc and Merle.
Merle Travis... when you invent a playing style that influenced the likes of Doc and Chet you belong on this list and one can only imagine if he used more than two fingers on his right hand.
Chet Atkins... he took things far and beyond and that fellow he is playing with might be a little under-rated.
Les Paul... electrifying the world.
Leo Kotke... don't try this at home.