Just to be clear, I also would have accepted the Beatles in Hamburg, Dylan in Greenwich Village, c. 1961, Duke Ellington at the Cotton Club, c. 1928 and the 1808 concert in Vienna which saw the premieres of Beethoven's 5th and 6th symphonies and 4th piano concerto.
I'm pretty sure I caught Beethoven in Vienna in a previous life but it could've been Mozart.