T/here have been a lot of great duets over the years... Elton John and Kiki Dee, Stevie Nicks and Don Henley, and Stevie with Tom Petty, Queen and David Bowie and the list goes on. Here are three of my favorites.
Sheryl Crow and Kid Rock
Both of them on their own have voices I like, and combined just makes it twice as good.
Iggy Pop and Kate Pierson
I admit I'm not a big Iggy Pop fan but I could listen to Kate Pierson's voice any time. The two combined, though work really well together
Aaron Nevile and Trisha Yearword
Most people are more familiar with Aaron's duet with Linda Ronstadt, "Don't Know Much". This one with Trisha Yearwood from the Country, Rhythm and Blues album is equally good. In the late 80's Aaron came in to a place I was working and it was amazing to see how big of a guy he was yer had such a soft, silky voice. It's on full display and syncs very well with Trisha's.
Huey Lewis and Gwyneth Paltrow
I first heard this when I watched the movie. Any movie with Paul Giamatti and Andre Braugher has to be worth watching. And any song with Huey Lewis is worth listening to. This cover isn't as good as the Smoky Robinson original but few covers match up to the original. Still it's a fine effort.