Technically and emotionally, Mariah wins this one. But there are many songs, from people who are not known for their powerhouse vocals, that are a nightmare to interpret. When you try singing some of these songs, you understand why they have a career, in the first place, and the difficulty of pulling off what they pull off.
She made that sound so easy Emotions was the first song that came to my mind. I Will Always love you isn't that hard technically, but nobody has been able to do it like Whitney. Some other pretty hard songs that come to mind are Heart Attack by Demetria Lovato (F3-G#5), Behind These Hazel Eyes (F#3-F#5), Since U Been Gone (G3-G5) and Honestly (G3-G6) by KC.
Recently Problem is a bitch and Chandelier sits up there as well (but Chandelier isn't impossible to sing, the highest belt is an F5 and the highest note is an F#5).
Heart Attack by Demi comes to mind. Lots of big notes & very little time to catch your breath. You have to have excellent breath control to pull it off.