Best..? Maybe the very first: the Godshill IOW 'festival', 1968. Seeing Jefferson Airplane, Fairport Convention, Pretty Things, Blonde On Blonde and more... 25s entry (that's £1.25 in modern money...). Splendid. Many others; too many to list, but that first exposure was the start of... Well, everything, really.
Worst..? Quite a few, since, but special mention for The Ox (Brunel University...), playing dreadfully, dreadfully loud. Mention, too, for so many concerts and festivals spoilt by the (at the time...) omnipresent Edgar Broughton Band.