When it comes to categorizing what makes a great heavy metal vocalist, there are a lot of factors that come into play. First off, there's the matter of subgenres. Mainstream, Swedish, death, black, melodic -- as much as I hate to pay attention to this bottomless list, it's a fact of the industry.
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And then there are the standards by which you measure a great metal singer, typically including power, vocal range and variation in tone. It also matters if you like clean melodic singers (Rob Halford of Judas Priest, Dave Draiman of Disturbed), or harsh growlers (Amon Amarth's Johann Hegg, At The Gates' Tomas Lindburg).