Evan Hansen is kind of a dangerous role. If the performer isn't staying safe, they can cause permanent vocal damage. The part is very hard to sing, requires extreme emotion, and is on stage nearly the entire show. The age of the actor is also a factor. Any man under the age of 35 has to be cautious because their voice has not fully matured. This is when it is the most susceptible to damage. All in consideration it is probably the hardest role to sing on Broadway right now.
Elphaba is also an extremely difficult part. I actually knew a woman who was cast early in the run when she was 23 and turned the role down because every teacher or vocal coach that she contacted about it, told her that it was a vocal killer and that if she were to perform that role 8 times a week at her age, her voice would never sound the same after. She's created a very successful life for herself regardless and is on Broadway right now, but she probably wouldn't be if she had taken that role way back when.
Some of hardest of all time: Eva Peron, Jean Valjean, Fanny Brice, Mama Rose, Caroline Thibodaux, Hedwig, Sweeney Todd, Nellie Lovett, and Eliza Dolittle.
I would say Christine and the Phantom, but so much of their music is prerecorded so I'm not counting them.