Freddie Mercury (Queen):
He had such a great voice, and was able to sing in a variety of ways that many after him only could try to emulate (for example, how similar to Elvis he sounded in Crazy Little Thing Called Love, or how soft and beautifully he sung in Bohemian Rhapsody)
Robert Plant (Led Zeppelin):
Probably the most soulful singer I have ever heard. He put so much energy and soul into his singing that it put him in a class all his own. He could howl on one track and the next he's singing in a softer, more toned down voice. Great voice for Zep's blues tracks too imo.
Eric Burdon (The Animals):
Burdon had that great, deep voice that sounded so hauntingly good. He just seemed to have the right tone to make songs like House of the Rising Sun sound even more daunting and memorable.
Jim Morrison (The Doors):
Jim had the best voice for his time imo. He had a soothing, sultry voice, that, like Eric Burdon, was nice and deep. The calming effect it had really suited well to The Doors' music too.
Don Henley (Eagles):
The thing that makes Henley's voice so great is its raspy, gravelly sound. It's subtle, but enough to make a significant impact on how each word he sung was heard and felt.