Cobain had the most perfectly inperfect voice I've ever heard and would go as far to say he's in the top 3 male vocalists of all time, he really doesn't get the credit he deserves in that respect.
Not sure about top male vocalists, that would depend on how you define something like that. I don’t think many singers get close to Frank Sinatra in terms of both technical and stylistic range. Frank literally had it all. But for emotional intensity I’d put Cobain up there with the likes of Jim Morrison and Otis Redding.