Well I voted for Soundgarden, Here is why:
Vocals: Chris Cornell sings like an angel one minute, and like a madman in the next, his highs and lows give me sivers when I hear them, he clearly out powers and has a better range than any of the other "Grunge" vocalists. IF you want "proof" listen to "Slaves and bulldozers" from Badmotorfinger.
Guitar: Tyail is simply a badass, no one can touch him coming from this poll, Black hole sun solo is an amazing piece of music. His unique blend of punk, hard rock and psychedelia is masterful, and ver unqiue compared to the other bands.
Those are two big reasons but if i need to go further i will. I have all of Nirvana's cds, decant but pretty weak compared to the other grunge bands. AIC, I have Dirt and greatest hits, i really don't like the lead singers voice that much, but other than that pretty good. STP, i have all of their albums, the only weakness is the some songs sound the same as some others. Pearly Jam, I have Ten, nothing too unique to me about them except Vedder's voice. So if i had to rate them:
Stone Temple Pilots