Example. "People Will Say We're In Love." First of all, the version with Hugh Jackman is just mind-blowing, because Hugh Jackman is stunning and perfect for that role. But even the song itself is brilliant. It's funny and sweet and touching. It rocks. (See, R&H, you had it in you! Why didn't you put a nice song like that in South Pacific? Did you use all your creative talents on "This Nearly Was Mine"? Because that's not a good excuse!)
Or what about "In Whatever Time We Have"? From Children of Eden, a slightly lesser-known musical, which I was lucky enough to be in during freshman year. That song has the most beautiful harmonies, and the two people that sang it in our show sounded great together. That song is so beautifully composed, and oh my goodness I love it so much.
And hey, a good duet doesn't need to be a mixed couple. Just look at "In His Eyes," one of my absolute favorite musical theater songs. It's from Jekyll and Hyde, which is an awfully dark musical, but very beautiful. My voice teacher is a big fan of Jekyll and Hyde. I know one girl who is her student also and has sung at least three songs from Jekyll and Hyde over the course of two years. As far as I'm concerned, they're all worthwhile, but if you watch the Hasselhoff version, some of his songs are awkward. It's hard to appreciate a song when the singer has no neck. I don't know why, but that disconcerts me rather.
And if boy/girl duets are how you roll, there's another in Jekyll and Hyde that I have been known to listen to on repeat for hours at a time. (Not exaggerating.) "Take Me As I Am." It's breathtaking here (although the lady that plays Emma is a little odd looking. She has a great voice, though.) and you should listen to it. Possibly my current favorite duet.