1. Daniel Gildenlow - (Pain of Salvation....one of the most unbeleivably strong and VARIED voices in history. This man can litterally do anything with his voice, it's just unreal...oh yea...and he's absolutely flawless live. THe song "Reconciliation" is probably the most concise way to show off his voice and how he can do so many things so perfectly (although definitely not the best)). He also writes almost all the band's music. I just can't say enough about how powerful, moving, and absolutely awe inspiring this guy's voice is. Just listen to "Reconciliation", you'll understand. Oh yea...he's got an incredible sense of melody on the guitar and he plays every instrument known to man.
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2.Dan Swano - (Best death metal voice in the biz, imo. Also a very good "normal" singer)
3.Mikael Åkerfeldt - (Opeth. Great mixture of a very good singing voice with an even better death metal voice) (glad somebody mentioned him so i didn't have to track down that weird A)
4.Chris Cornell (I think his works have already been mentioned enough. Not my choice of music but he's definitely got a great voice)
5.Bruce Dickenson (Iron Maiden, Bruce Dickenson - does anything really need to be said about him? I mean...he's Bruce Dickenson)
EDIT - Oh god I forgot Mike Patton, put him in at #2 and shift everything else down 1.