While they may not be the greatest, here are some of my favorites:
James Hetfield (early Metallica preferrably, esp. Kill 'em All)
David Gilmour - Pink Floyd
Roger Waters - Pink Floyd
Chris Cornell - Audioslave, Soundgarden
Mikael Akerfeldt - Opeth
Robert Plant - Led Zeppelin
Maynard James Keenan - Tool, A Perfect Circle
David Draiman - Disturbed
Benjamin Burnley - Breaking Benjamin
Lead singer from Dream Theater (can't think of his name)
Cristina Scabbia - Lacuna Coil
I'm sure there are more, but those are the only ones I can think of right now.