Doc has so many albums - over 45 or 50 - many of which are slightly overlapping in content or are compilations of prior recordings.   Doc recorded too many good tunes and too many tunes of radically different styles to have one cd of greatest hits that everyone agrees on. I picked the ones I bought simply because they have some of the specific tunes I wanted on them - Tennesse Stud, Salt Creek, Blue Railroad Train, Shady Grove, and Fishers Hornpipe just to name a few.   But everyone has their own favorites

You may want to go to some of the dscography sites and look over the choices - docsguitar.com is a pretty good one.

Some good choices might be Vanguard's 4 cd set Doc watson The Vanguard Years, or the Best of Doc Watson 1964-1968 or The Essential Doc Watson also by Vanguard. Tennessee Stud put out by Music Mill and Watson Country by Flying Fish also have some good represenative sample of his work, but you really cant go wrong by just randomly picking something