My all-time favorite country duet would have to be "Come On In" by Brad Paisley feat. Buck Owens. The greatest thing about that song is that it was used for Paisley to pay tribute to Buck Owens, even though Owens had written it in tribute to his guitarist. Also, it's great to actually hear one album from 2008 with an actual country song on it (especially on an album full of multiple genres with a lot of great guitar work and one that debuted at #1)!
Some of my other favorite country duets are:
"Designated Drinker" by Alan Jackson feat. George Strait
"Never Loved Before" by Alan Jackson feat. Martina McBride
"House of Cash" by George Strait feat. Patty Loveless
"Sneakin' Around" by Chet Atkins feat. Jerry Reed
"Whiskey Lullaby" by Brad Paisley feat. Alison Krauss
"Out in the Parkin' Lot" by Brad Paisley feat. Alan Jackson
"More Than Just This Song" by Brad Paisley feat. Steve Wariner (another great song paying tribute to musicians...one person who this pays tribute to is Paisley's guitar teacher, Clarence "Hank" Goddard." The other guitarist this song pays tribute to is Steve Wariner's mentor and friend, Mr. Guitar, Chet Atkins)