I know that Morricone was annoyed about how much praise his western scores received compared to his other work, but I can't help but thinking that most of his best non Leone work was in the spaghetti subgenre, at least for my music taste:
Actually, outside of Leone, his other greatest partnership must have been with Giuseppe Tornatore.
I wasn't talking about director partnerships though -- and I would take Morricone's non Leone spaghetti work over this Tornatore stuff in a second, even if that's the second best partnership. But it's such a distant second that I don't think that it means a whole lot.