Dave Mustaine - one of the best angry snarlers ever. His high range has gotten weak with age, but he still kills it.
Halford/Dickinson - Gods. Untouchable.
Phil Anselmo - metal's most badass voice.
Doug Pinnick - fucking brilliant.
Ian Gillan/Robert Plant/Phil Mogg - their importance simply cannot be overstated.
Warren Zevon - strange, moody, genius.
I could go all day with this.