Whatever you can say about Nevermind and In Utero, for me this is Nirvana's most original record. That's not to put the others down - Nevermind is a fantastically energetic and powerful album, while In Utero is by turns elegant and degenerate despite remaining about as emotional as you can get. I'm not going to talk about Bleach btw.
Anyway, Insesticide while not always easy to listen to has more passion than anything else Nirvana have done and at times Kurt's voice sounds clearer and more agonised than I've heard it elsewhere. His vocals are varied throughout, shifting through paranoid sounding mumbles, to piercing screams and aggressive grunting through to sweet tunes along the way.
Obviously a rough recording at times and not likely to remain in your cd player as much as the two major records but for raw talent, emotional fire and unusual rock inventiveness, this is a good place to come.