I'll second "Baby Ride Easy" by Carlene Carter and Dave Edmunds with the famous lines:
If your lovin' is good
And your cookin' ain't greasy
HITCH UP the chuck wagon
And we'll ride away
Kasey Chambers & Shane Nicholson - Wreck & Ruin " Adam and Eve" love that song!
Dailey & Vincent
Shaver - Tramp on your Street " Heart of Texas & Oklahoma wind" with Waylon
Tim O'brien & Darrell Scott - real time
"The Quiet Life" is my favorite duet from Wreck & Ruin.
I love Ernest Tubbs big grin in the background!
One of my favorites is Kelly Willis and Robbie Fulks doing Parallel Bars.
I wanted to mention "Parallel Bars" also, but I didn't know who sang it. Thanks for all you do for ND.
Sticking to your criteria of Country duets with great harmonies may I suggest:
No One Is Going to Love You Better - Heather Myles and Merle Haggard
How's the World Treating You - Alison Krauss and James Taylor
You and Me and Time - George Jones and Georgette Jones
My Kind of Woman - Vince Gill and Patty Loveless
The New Partner's Waltz - Linda Ronstadt and Carl Jackson
Love and Happiness - Mark Knopfler and Emmylou Harris
I heartily recommend you check out My Darling Clementine for a batch of original songs written and performed by a UK duo, including these.
I love Lyle Lovette and Francine Reed
The first thing that came to my mind though was Johnny and June doing Jackson. Another that I haven't noticed yet is Sonny and Cher doing "I've Got You, Babe". Am I too old and conventional?
Willie and Waylon doing "Pancho and Lefty" is also a favorite.
Okay, Gillian....I've read through a lot of these great choices, but I showed up late for the dance - so if this has been said already, I apologize. My vote goes to Fairytale in New York with Shane MacGowan and Kirsty MacColl, written by MacGowan and Jem Finer. The song breaks my heart every time I listen to it.
Too many to choose from. Here's a list of some of my favorites. There are a few repeats from earlier lists and some additions:
Love Hurts - Gram Parsons & Emmylou Harris
The Battle - Ryan Adams and Caitlin Cary
Conversations About A Friend (Who's in Love With Katie) Caitlin Cary & Thad Cockrell
Keep Your Distance - Buddy & Julie Miller
Oh My Sweet Carolina - Ryan Adams and Emmylou Harris
Juanita - Sheryl Crow & Emmylou Harris (Gram Parsons cover, from Return of the Grievous Angel tribute album)
The Wedding Song - Charlie Robison & Natalie Maines
Golden Ring - George Jones & Tammy Wynette. I actually like the version they did in 1994 on George Jones' Bradley Barn Sessions album.
Whiskey Lullabye - Brad Paisley & Allson Krauss
God's Wing'ed Horse - Buddy & Julie Miller
A Song for You - Whiskeytown (Gram Parsons cover, from Return of the Grievous Angel tribute album. It's not officially a duet, but Caitlin Cary's harmony is so perfect on this)
Little Chapel - Dwight Yoakam & Heather Myles
Warm and Tender Love (Percy Sledge cover) - Caitlin Cary & Thad Cockrell. This may be my favorite of all the ones I've listed. Here's a performance on you tube, but this pales in comparison to the album version from "Begonias."
I don't think Oh My Sweet Carolina's really a duet either , is it ? I've certainly never thought of it as one. Not that it matters mind you .