Frank Mullen (Suffocation)
Karl Willets (during Bolt Thrower)
Nocturno Culto (Darkthrone)
3 is pretty limited, but these guys just hit the sweet spot for me. Others are Ross Dolan (Immolation), Lord Worm (Cryptopsy), Craig Pillard (Incantation), Chalky (Psycroptic), Antti Boman (Demilich), Rain Pohlak (Neoandertals), Dave Ingram (Benediction, Honour Valour Pride), Martin van Drunen (Asphyx, Pestilence, Hail of Bullets), Chris Reifert (Autopsy, Abscess), Daniel Corchado (The Chasm, Diabolical Conquest), Eric Harrison (Grief), Matti Way (Disgorge), A.J. Magana (Defeated Sanity, Consume the Forsaken), Vlad (Mordeth), Danny Nelson (Malignancy), Will Rahmer (Mortician), Dennis Schreurs (Severe Torture)...