probably Jesus Christ Superstar, however I'm a fan of musicals (oddly enough)
Grease!, Little Shop of Horrors and of course Les Miserables are high on the list.
Cats? ..meh, I don't 'get it' why it's so popular.. a snoozer for me.
(And though it's not "quite" a true musical per se, the Rocky Horror Picture Show is a personal fav.)