Gustav Mahler. I find him to be far too obsessed with details so that I never am able to see the full-effect/big picture of the work. And I am sorry, but a 95 minutes symphony is simply not necessary.
I do also agree with Vivaldi.
Musical theater wise, and I think this probably goes without saying, ALW. His work is very pedestrian. I can't say that's entirely negative, part of what makes him so popular is that is music is very accessible to a listening novice.
"You just can't win. Ever. Look at the bright side, at least you are not stuck in First Wives Club: The Musical. That would really suck. "
--Sueleen Gay