I would say for someone new to extreme types of music:
Slayer - Seasons in the Abyss
Kreator - Violent Revolution
The Haunted - ...Made Me Do It
Dew Scented - Inwards
Entombed - Clandestine
Morbid Angel - Domination (always my favouriote M.A album)
Nile - Black Seeds of Vengeance
Cryptopsy - None So Vile
Opeth - BlackWater Park
Dark Tranquillity - Projector
Death - Any album
Cannibal Corpse - Eaten Back To Life
Carcass - Heartwork
Soilwork - Natural Born Chaos
Emperor - Prometheus: The Discipline of Fire and Demise
Darkthrone - Under a Funeral Moon
In Flames - The Jester Race
Marduk - Panzer Division Marduk
Zyklon - World ov Worms
Mayhem - De Mysteriis Dom Sathanas
Children of Bodom - any album
Arch Enemy - Black Earth(?) or Wages of Sin
That should be a good start