I achieved a long time ambition by seeing Doc live in Burlington VT, about eight, nine years ago. His guitar playing and singing were just as good as ever, although one tiny caveat for me was that I would have preferred his guitar to have sounded more acoustic and less "electric", but that is more of an indictment of the sound engineer than of Doc, I imagine.
My favourite Youtube video ever is Doc Watson singing and playing Deep River Blues . It shows off his amazing voice to perfection, and also highlights his ability as a fingerpicker, for which he is less well known than his flatpicking.
This video has been pulled repeatedly, presumably for copyright reasons, but it is up there at the moment.