At the risk of turning this thread into an homage of great acoustic/bluegrass guitar players, I'll go ahead anyway and mention Doc Watson. I haven't listened to him in years. God is he smooth.
You can't put enough "O"s in smooooooooth to describe his playing. I got to see him a bunch of times. Several years as his "Merle Fest," and handful of shows including an awesome one at our local theater in Blacksburg. the best was at Wayne Henderson's Christmas Party at Wayne's house in Rugby, VA. Just a house party, no PA, just folks sitting around the kitchen and his shop playing some amazing music how it was meant to be played. I probably could've joined in but was happy just to listen. What an amazing human being he was.