My greatest concert was also Bowie.
Close runner-ups would be Kraftwerk, David Byrne, Devo, The Cure, Al Green, Sonic Youth, The Pixies, Björk, and Tom Petty and the Heartbreakers. I think I really liked Radiohead when I saw them, but then I kinda got over 'em.
And the greatest concert I've ever been to for non-musical reasons was Rage Against the Machine and Ozomatli protesting outside the Democratic National Convention in Los Angeles in 2000. I got kicked in the head in a crowd, and then when the cops went nuts we got teargassed and they shot rubber bullets at us and we ran like the lightning. I ran right past the leader of the National Homeless Coalition, and then that night on the news saw that the cops shot him in the neck and almost killed him.
I'd kill to see Billy Joel though. Have you guys heard of Atilla?