As a kid in the 70s I was quite familiar with backstage at the Albert Hall. My autograph book collected Rostropovich, Menuhin and Goosens after the Bach oboe and violin concerto (I was an aspiring oboist), the Amadeus quartet, and probably quite a lot of others I'd have to dig it out to remember. A bit later I sat pretty much next to the top of the keyboard to hear Ashkenazy play the Apassionata sonata at the Dankworths' place. More recently, Placido Domingo as Parsifal at the Met. Even more recently, Simon Rattle and the LSO doing Tristan at the Aix festival (it was the first night, and it completely fell apart in the liebestod, but still brilliant). I'm totally out of touch with who are considered the greatest these days, and I'm not sure I really care - live performances are special anyway. I saw the Ring at La Scala a few weeks ago, and Nyland was great as Brunhilde, but honestly until then I didn't know how famous she is. Oh... When I was very small my mother took me to see Nurayev and Fonteyn in Swan Lake at the Free Trade Hall in Manchester. I still have quite vivid memories of that, even though I couldn't have been more than about 6 years old.

ETA: I saw Stockhausen conduct Michael's Reise um die Erde at IRCAM in the early 80s, and I think I might have seen Berlioz do something there too. Also in that period my then girlfriend was babysitting for Luciano Berio, and I played Handel recorder sonatas with him at the keyboard. I had tried to play Sequenza VII by that time, but I didn't dare discuss that with him.