Best guest backing vocals in rock/pop/rap/whatevs music

W. Axl Rose on Don Henley's "I Will Not Go Quietly".

And of course Axl's guest appearance at the Freddie Mercury tribute....

 

The Fairfield Four (and, yes, I Googled it) on John Fogarty's "110 In The Shade"

 

Then here's an SCTV clip you'll appreciate:

 

I think Linda Ronstadt sings background on that one as well.

A couple more:
Joni Mitchell on the Last Waltz version of "Helpless"

Flo and Eddie from The Turtles on virtually every T. Rex song

Paul McCartney on "Mellow Yellow" and "Atlantis" by Donovan

 

--Beach Boys, Wishing You Were Here by Chicago
--Patty Smythe, Where Do the Children Go by the Hooters and Sunset Grill by Don Henley

 

One I'm surprised hasn't been mentioned already: Sting's "I want my MTV" on Dire Straits' "Money for Nothing."

 

Crap, surprise indeed. Someone mentioned Nicks. How about Nicks and Buckingham on Magnet and Steel by Walter Egan

 

Bruce Springsteen on Warren Zevon's "Disorder in the House" (or would that be a duet vocal, not a backing vocal)

 

Bruce Springsteen on Joe Ely's "All Just To Get To You"
Live version:

 

I don't know that you'd say "best," but Michael Anthony's backing vocals for Van Halen were a pretty key part of the band's sound.

 

T-Pain in everything.

Edit:

#t=1m05s

 

Anthony was a member not a guest