Glitter and Be Gay is definitely up there. Forgot that the first time. And Green Finch and Linnet Bird is really tough as well. Getting Married today is tough just to sing that fast without getting tongue-tied. Your Fault really isn't that hard. You just need a good music director.
If we're going for hardest physically/emotionally, I would most definitely say "And I Am Telling You" which is just high notes after high notes with emtional breakdowns and more vocal pyrotechnics. Many said that Jennifer Holliday would faint offstage after singing that song. Also hard physically is a lot of the stuff in Brooklyn, just because it is so high AND it's belted. For a song to end on a high F belted, you can't find many women that can do that 8 nights a week.
Eeeeeeyyyyyyyyaaaaaaaannnnnddddd aaaaaaaiiiiiiiiyyyyyyaaaaaammmmmmmm teeeeeeeelllllliiiiiinnngg yyyyooooooouuuuuuuwwwaaaahh...