I would say:

Metallica

Kreator

Megadeth

Judas Priest

Slayer

Testament

Tankard

Helloween (1st Power Metal band)

Dio (Progressive Rock, not metal but a really good style)

Rainbow (Progressive Rock)

Dream Theater (Progressive Rock)

Anthrax

Metal Church

Cannibal Corpse

Death

In Flames

Opeth

Overkill

Exodus

The Sword (heavy metal, alternative metal, but still pretty cool)

Symphony X

Moonspell (Portuguese band)

Motorhead

Diamond Head (1st album is awesome)

Iron Maiden

Black Sabbath

Led Zeppelin (best band ever, in my opinion)

Pantera

Annihilator

Arch Enemy

Machine Head

Rush (Hard Rock, Progressive Rock)

AC/DC (Hard Rock)