"Barcelona" - OK, not your normal duet, but I love the way Sondheim weaves those dangling back and forth unfinished sentences. So deceptively simple, yet so sophisticated.
Ditto: "I Will Never Leave You" (as good as it gets in the modern era, imho)
Another vote for Twin Soliloquies. Also, I bought a great CD that's all Broadway duets of love songs. Wonderful!!! Here's the link. I got it at Target. Such great voices too, except for "It Only Takes a Moment" but that song is so beautiful it doesn't matter. My favorites after listening a lot turn out to be the older couples, Mary Martin and Robert Preston singing "My Cup Runneth Over" and Zero Mostel and Marina Karnilova on "Do You Love Me." Would have been really cool if Zero Mostel and Mary Martin would have performed together, but guess that wasn't going to happen, but their voices have so much character. Amazon has it too, their page lists all the songs, though they've botched the artists http://www.amazon.com/Falling-Love-Is-Wonderful-Broadways/dp/B000DZ7Y5Q/ref=sr_1_1?s=music&ie=UTF8&qid=1340161711&sr=1-1&keywords=broadway+duets Ethel Merman and Bruce Yarnell sing "They Say It's Wonderful" Yarnell had such a great voice. Josie de Guzman and Peter Gallagher sing "I've Never Been in Love Before." Daniel Ferro and Jeanette Scovotti sing "We Kiss in a Shadow" Jo Sullivan also sings "My Heart is So Full of You." Other weird names listed, they are all just duets. Love them all except the modern ones- 13 & 14. Guess I just love the classics. I'd put all these on my list, except 13 & 14. Great songs, great voices! Falling in Love Is Wonderful: Broadway's Greatest Love Duets
I would like to throw a couple of other ideas out there as well. These are just duets that I love listening to.
What Do the Simple Folk Do?, Camelot If I Loved You, Carousel Tonight, West Side Story Say It Somehow, The Light in the Piazza Move On, Sunday in the Park with George The Riddle Song, Floyd Collins Honeysuckle Rose, Ain't Misbehavin' What You Don't Know About Women, City of Angels Agony, Into the Woods If Momma Was Married, Gypsy You Were Dead You Know, Candide Who Could Be Blue/Little White House, cut from Follies Last Chance Blues, Jelly's Last Jam Freedom, The Unauthorized Autobiography of Sam Brown Come Up to My Place, On the Town There Is No Other Way, Pacific Overtures The Picture Show, Parade The Wheels of a Dream, Ragtime What a Piece of Work is Man, Hair (I got a little carried away)
I also add my support and surprise that the initial list didn't include A Little Priest