For pure talent, not surprising I suppose, it was Doc and Merle Watson (late '70's) at a really small venue in Memphis, TN. Just the two of them. Super clean, awesome acoustic guitar, crystal clear vocals, and Doc pausing at one time between sets: "where's my A?" - as he dug through a selection of harmonicas - "ah - mint". He then went to explain that (since his "seein' wasn't so good" he flavored his harmonicas so he could "tell 'em apart".