Cornell, because he's the best singer of the three. And Soundgarden never released a bad album.
Staley was pretty good. His voice gets annoying after a while, which is why I always preferred Jar of FLies and Sap. Dirt is good, but everything else is horrible, like Pearl Jam's entire discography and Eddie Vedder's clench-teeth vocal delivery.
Nirvana had a few shining moments on Nevermind, but other than that, awful.