Top 10 Songs with the Best Rock Singer Collaborations

Songs where rock / metal singers sing with singers from other bands or solo artists.
Singers from bands with two lead singers or backing vocalists from the same band don't count.
Guest apearances, side-projects and supergroups are OK.

At least one of the singers has to be a rock or metal singer but collaborations may include singers from other genres - opera, pop, rap, jazz, blues, ...

The Top Ten

1 Barcelona - Freddie Mercury & Montserrat Caballe

"How can I go on" is almost as good as Barcelona.

2 Age of Shadows / We Are Forever - Ayreon

3 All for Love - Bryan Adams, Rod Stewart, Sting

4 Smoke on the Water - Deep Purple, Queen, Black Sabbath, Iron Maiden, Pink Floyd, Yes, Rush

Vox - Ian Gillan, Bruce Dickinson, Paul Rogers, Bryan Adams.
Kerrang called this collaboration "the greatest array of hard rock talent ever assembled" (including guitarists, bassists and drummers, and not only singers).
This song is on YouTube.

5 We Don't Need Another Hero - Northern Kings

6 The Scarecrow - Avantasia

7 The One You Love to Hate - Rob Halford

The song sample is from a live version because I didn't find the studio version on iTunes.

It's a duplicate - it's already in the top 10

8 Let the Storm Descend Upon You - Avantasia

9 Out of Control - Edguy

Tobias Sammet and Hansi Kursch.
I also think that one line in the background is sung by Timo Tolkki who was a guest guitarist (but I know he can sing). Just the voice on that line doesn't sound like Tobi or Kursch to me (it's on the song sample - "and let me stay blind").

10 The Seven Angels - Avantasia