Your "Ultimate" Collaboration?
No more than five individuals.
Mine would be Thom Yorke, DJ Shadow, Trent Reznor, RZA and Clint Mansell.
How fucking awesome would that be.
I mean, Thom and Trent on vocals, Trent and Shadow on music (instrumental and digital production and reproduction of said instruments, respectively), Mansell on writing and strings arrangement, and RZA as executive producer, with Shadow, Mansell, and Trent co-producing.
I mean, it would work perfectly.