re: Kurt Cobain had the worst vocals of the grunge bands

Not even close. If Kurt sucked so much, then why was Dave Grohl on backup vocals? Most of the other grunge bands you didn’t list had worse lead singers.

Of the four bands you listed, he is three or four on vocal talent but one, two, or three as a great lead singer. His style fit the band, their genre, their band, and the band’s terroir.

I personally rate early Cornell #1 in vocals (“Birth Ritual”, etc.). Staley was amazing, but mostly too drugged out, and therefore ended up being a great compliment with Cantrell. While Vedder was the drunk / less interesting Cobain. I love them all, but they all have/had their deficiencies.