... I have to pick one??
Most of my favorite musicals can probably qualify as guilty pleasures, as I am a huge fan of the classics. It may not be as cultured as saying I love Sondheim (which btw, I do!) or Kander and Ebb (again, love!) but stuff like Guys and Dolls and Kiss Me, Kate... well, their classics for a reason!
But for a real guilty pleasure, I'd have to go with... *shiver* Les Miserables. It's horrifying to many of my friends, but I love it. I could just sit and listen to the soundtrack for hours... and I always cry when Fauntine dies... and at the end.
So, yeah, my guilty pleasure is really only a guilty pleasure because it's so mainstream!