Many of the greats have already been named (Halford, Dio, Tyrant, Eric Adams, etc.) and I'd like to throw a few more out there.
Daniel Heiman (ex-Lost Horizon)
This guy is absolutely ridiculous. He has a great mid-range voice and he throws out high-pitched shrieks all over the place. I don't care for his new band (Heed), but Lost Horizon released two great albums with him on vocals.
http://media.putfile.com/Lost-Horizon---Heart-of-Storm
http://media.putfile.com/Pretty-Cool
Iuri Sanson (Hibria)
Just an overall solid voice with fantastic range and a great band backing him up.
http://ursvamp.powweb.com/hibria/hibria__steel_lord_on_wheels_2004.mp3
John Cyriis (ex-Agent Steel)
I don't think anyone knows where this guy is, but his work on the first two Agent Steel albums absolutely rocks if you like high-pitched vocals in your speed metal (who doesn't?)
http://media.putfile.com/Agent-Steel---Agents-of-Steel
Midnight (Ex-Crimson Glory)
Crimson Glory's ballads pretty much sucked and I only really enjoy their first two albums, but nobody can deny Midnight's prowess on vocals.
http://media.putfile.com/Crimson-Glory--Valhalla
James Rivera (Helstar)
Not only did Helstar have some of the most tasteful lead guitarists in their prime, but they also had this guy's great pipes.
http://media.putfile.com/Winds-of-War
I also really enjoy Jari Mäenpää's (Wintersun, ex-Ensiferum) clean vocals that he uses when he doesn't use his black metal-esque rasp. They are the soaring type that, while not particularly challenging, are very appealing to me.