Oddly enough, I don't think of Maynard's voice as "metal" at all.
Yeah, but Maynard's voice is awesome, but being a tenor he still has a good range but isn't really a suited metal voice, so gentle and controlled but really driven if he pushes his singing (yes, actually sings)...he provides a good atmosphere to the band (someone used a sentence like that before)...it wouldn't be Tool without him.
Four best metal voices in my opinion:
1.Serj Tankian from SOAD
2.Jens Kidman from Meshuggah
3.Tobi from We Butter the Bread with Butter (OMG brutal pig squeels...BBRREEKACHUU!)
4.Brian Fair from Shadows Fall