David Bowie and Marianne Faithfull doing the classic Sonny and Cher's song I Got You Babe.

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A Classic Duet.

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Ozzy Osbourne and Type O Negative - Pictures of Matchstick Men
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I forgot about the entire Judgment Night Soundtrack. That one was a lot of fun.

Finally, a chance to express my love of this duet!

John Lithgow & Bebe Neuwirth - Baby It's Cold Outside

Scarface & Ice Cube - Hand of the Dead Body

Ozzy Osbourne & Miss Piggy "Born To Be Wild"
Alice Cooper & The Bee Gees "Because"
David Bowie & Mick Jagger "Dancing In The Streets"
The KLF & Tammy Wynette "Justified And Ancient"

nick cave and shane macgowan - what a wonderful world
nick cave and kylie minogue - where the wild roses grow
and of course, as mentioned, fairytale of new york.

David Byrne & Natalie Merchant - "Let the Mystery Be"

Neil Diamond & Barbra Streisand - You Don't Bring Me Flowers

Other faves:
Sarah Brightman & Andrea Bocelli - Time to Say Goodbye
Nick Cave & Kylie Minogue - Where the Wild Roses Grow
Nick Cave & PJ Harvey - Henry Lee
PJ Harvey & Thom Yorke - This Mess We're In
PJ Harvey & Bjork - Satisfaction
Bjork & Thom Yorke - I've Seen It All

Mariah Carey & Boyz II Men - One Sweet Day
Still gives me goosebumps. Boyz II Men is my favorite musical group, ever.
http://www.youtube.com/watch?v=nzeMzo_4pxA

Queen & David Bowie - Under Pressure
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Mos Def, Talib Kweli, Common - Respiration
OK, so I cheated a bit on this one, but it's too good not to post.
http://www.youtube.com/watch?v=vHReQQnMVQo

Forgot these.

Jay-Z & Eminem - Renegade


Scarface, Bushwick Bill, Ice Cube, Kool G Rap - Two To The Head

This had no right being as good as it turned out:

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This is one of my favorites, Morrissey and Siouxsie:

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Elton John and Eminem - Stan

Maybe musically not the greatest, but it was really cool with all the crap Eminem was getting from gay rights groups at the time.

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This would be my pick as well. Bowie's vocals complemented Mercury's so well, and the song is simply amazing.

Michael Stipe and Coldplay covering "In the Sun". I'm not sure if this would count under the OP's rules, since they also dueted on "Nightswimming", but I'm including it anyway.

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Jackie Chan and Ani DiFranco- "Unforgettable"

The Winner is:George Michael/Aretha Franklin - I Knew You Were Waiting (for me)
Two of the Greatest Singers Of All-Time and no one comes close.

A bit triva on this song it was one-take each in the recoring of this gem for George and Aretha.

Not a duet.

Sting and James Taylor - "Fill Her Up" on the Brand New Day Album... a bizzare track... Starts as country outlaw song, then segues into a Gospel spiritual....

Peter Gabriel & Kate Bush - "Don't Give Up"



(I always liked video version 1 better)



Nena & Kim Wilde - "Anyplace, Anywhere, Anytime"





And how's this for "one-off"...Gary Numan and Leo Sayer singing "On Broadway" from Leo's show in 1984: