If you were to have a duet with with your favorite singer, which song would the two of you sing?

I'd love to do a duet with Idina Menzel! She's my favorite Broadway actress and singer, and one of the most inspirational women I've ever seen.
I'm a massive fan of symphonic metal bands like Within Temptation, and I think her powerful voice would fit perfectly. I chose the song from above because it's already a duet.