bands:
the Beatles
Queen
the Rolling Stones (saw them in '86 but would have loved to have seen them in their youth)
singers:
yes, I admit to a weakness for John Denver
and I felt so cheated when Harry Chapin died --- I was supposed to see him in concert that night, and he always did put on a great concert
Frank Sinatra -- especially in the 40's, before he got old and fat.
Peter Allen
Janice Joplin