Holy Terror's list has many of my faves as well (like Michaud per say), so I won't repeat them. Also I agree with Sweet Sister Mary and a few others. Here's some more picks :
Eric Adams - Battlehymn, talk about goosebumps...
Carl Albert - Kamikazee, it's from the first Villain album, the scream at the end of the song must be the highest octave any vocalist has ever hit... ever!!
J.D. Kimball - The curse album (by Omen), perhaps the most haunting vocal performance in the history of heavy metal, R.I.P. brother.
Tim Baker - King of the Dead, perhaps the second most haunting vocal performance blah, blah, blah!
Geoff Tate - Queen of the Ryche, talk about screams!
Midnight - Red Sharks (Crimson Glory from the transcendence album) talk about more screams!!
Tim "Ripper" Owens - Diamonds and Rust from the priest '98 live album, just because he can sing it as well as Halford
Harry "the Tyrant" Conklin - License to kill (from Jag Panzer's Ample Destruction album), the most in your face vocals you'll ever hear.
Anyway I can go on for hours, but just one more Sean Peck - Kill the Devil from Cage's Darker than Black album, simply deranged!
Oh and last but not least any Deadly Blessing song with Ski on vocals, damn that guy can scream!!!
Oh, oh and Halford - Painkiller, you didn't really think I'd leave the metal god out of my list did ya?