Tough one to come up with a best - I guess I've got several in a tie for first.
In no particular order:
Cream at the old RI Auditorium
Hendrix, same venue
Led Zeppelin at the Newport Festival
Sly and the Family Stone at Newport
Van Morrison, Fillmore East, and
Santana at Woodstock. Yeah, I was there but slept thru Hendrix...
Oh..from this century, Alison Krauss and Union Station at URI
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