This is a broad topic. My joke choice would be Stones at Altamont ’69. But here goes,
Mozarts subscription concerts in Vienna mid 1780’s where he introduced all those beautiful piano concertos and the premier of any of the Mozart/Daponte operas.
Any complete or nearly complete performance of Les Troyens during Berlioz’s lifetime.
The premier of Wagners Tristan und Isolde and/or Parsifal.
A Gilbert and Sullivan Operetta at the Savoy 1870-80’s preferably the Mikado.
On a more modern note,
The Beatles, Hendrix (on a not so sloppy night), Jeff Beck group w/Rod ’68?, The Who circa 1970 (probably Isle of Wight), Pink Floyd circa DSOTM, Zappa 73-74 with Roxy band and/or The MOI at the Garrick theatre ’67, Deep Purple at one of the Japan shows ’72.
Miles Davis, either great Quintet w/either Coltrane or Shorter (sorry Coleman), Louis Armstrong Hot Fives/Sevens and Duke Ellington (Blanton-Webster) circa 1940-42.