Interesting read. The comments are just as interesting, as the number of people trying to push Page/Plant can't know that much about Zep. Most of the songs are credited to the majority of the band, and so many of their songs are partial covers or reinventions of old blues tunes. Not to minimize the influence of LZ, but their skill was in performance and interpretation more than songwriting.

Mine would be Lennon/McCartney, even though we've learned that there really weren't all that many true collaborations

Lieber/Stoller

Jagger/Richards

John/Taupin - possibly the greatest duo since Bacharach/David or Rogers/Hammerstein or Rogers/Hart

Bacharach/David - These songs hold up so well.

Goffin/King are a close runner up, and if we had to get technical would replace Lennon/McCartney.

I'd also throw Bacharach/Costello in there for consideration. I think even their throwaways could be classics by the right performer.