All of this talk about ABBA and no one mentioned Chess! It has a horrendous plot (actually no plot at all) but the music is totally kickass.
I will always hold a special place in my heart for Bye Bye, Birdie because I met my boy when we did that show freshman year. He was Hugo Peabody and I was Deborah Sue, aka a 2-line chorus member.
My other favorites are Phantom of the Opera, Pippin, Tommy, Into the Woods, and Rent.
Random interjection, speaking of Tommy: Is anyone really surprised by this Pete Townsend child pornography thing? Hello, this is the man who created Uncle Ernie!