Ones which I don't think I've seen mentioned yet:
Bill Steer (helped to define grindcore AND melodic death metal)
Lee Dorrian (one of the great legends of grindcore AND doom)
Mick Harris (for helping to make the blastbeat what it is today, and thus pushing the boundaries of extreme metal)
Dan Lilker (Anthrax, Brutal Truth, SOD, Nuclear Assault, Venomous Concept etc)
Kevin Sharp (for being so badass)
Jesse Pintado (World Downfall, Harmony Corruption, Utopia Banished, Fear, Emptiness, Despair etc, nuff said)
Geezer Butler
Scott Burns (for producing so many defining albums)
Phil Anselmo
Rex Brown
Johnny Morrow
Pepper Keenan
Could go on for days...