Been to plenty in my day. Monsters of Rock at Donington, Reading Festival and various at Knebworth (which was walking distance from where I lived).
One of the most memorable was Slade playing at Reading in 1980. Ozzy Osbourne was booked to play but cancelled at short notice and Slade were booked 2 days before the gig. They had been going through a bad patch and the gig was intended to be their farewell performance but they absolutely stole the show. I remember Noddy Holder shouting at the crowd without a microphone, but you could definitely hear him right round the place. On their final encore they had run out of songs so asked what the crowd wanted to hear and everybody shouted "Merry Christmas Everybody" which they duly played on August bank holiday weekend!
Can't find any video of it but here is an audio clip.
http://www.youtube.com/watch?v=chsyGw6N_Is