"Getting Married Today" takes the cake IMO.
"Kiss Me" from Sweeney Todd is a mouthful, mainly because both parts are intricate and then putting them together. And Johanna's is all over the place in terms of range.
The Witch's part in the Prologue in Into the Woods. Also "I Know Things Now" and "Giants in the Sky" are pretty difficult but can definitely be done. "Maybe Their Magic" is not too difficult, but they did slow it down for the revival, so it must have posed somewhat of a problem.
"Act I Finale" in Urinetown is hard for an ensemble to get right. The individual parts are easy, but put it all together and the timing gets off and parts are hidden.
Dancing through life...
For the next ten minutes