I would say that Mayhem's current singer, Attila Csihar, is extremely unique. Unlikem some of their previous vocalists, who, for all their talent, were a little generic, Attlia is truely unusual. He sounds like the death rattles of a dying man. Listen to this if you want proof:
I'd say, in my opinion, Macabre takes the gold. They play, what they call, Murder Metal. http://www.youtube.com/watch?v=1M7-zXAiMM4 http://www.youtube.com/watch?v=f0NzVlej7xc&feature=related http://www.youtube.com/watch?v=T6FQ5bzROsU
There is also Disciples of Zoldon, the weirdest band I've ever heard. http://www.youtube.com/watch?v=WqvgF1c-DKU Everything he's ever done is available for free download at: http://www.solvere.co.nz/doz/Discography%20of%20ZOLDON.htm
Dani Filth of Cradle of Filth has pretty unique vocals. http://www.youtube.com/watch?v=ms9RQN8MaQ4 http://www.youtube.com/watch?v=ujeQBGS70lA
Kamelot has pretty unique vocals for a power metal band. http://www.youtube.com/watch?v=0Qdcl67H1Jw http://www.youtube.com/watch?v=58azvyFmKrc
Due it being an all female band, Astarte is a cool Black Metal band. http://www.youtube.com/watch?v=lPNKCpyAYlk http://www.youtube.com/watch?v=PcA_DWk2S1o
Vision Divine is another example, I guess. http://www.youtube.com/watch?v=kCRTPCsEEeU http://www.youtube.com/watch?v=4zYpmVfPsIw
Novembers Doom has weird vocals style. http://www.youtube.com/watch?v=Vqb3lbKqhFI http://www.youtube.com/watch?v=e0oQ7ZOKTls
I personally can't stand the vocals, but Converge is definitely unique. http://www.youtube.com/watch?v=utT0lLC6d5Q http://www.youtube.com/watch?v=oNfm6Flx1AQ