Thing is with metal, the bands that really changed or shaped the scene are the really big ones. I mean, metal is popular, and for every truely outstanding bands you get about a million clones (the only evidence I have of this, because of my age is during the whole nu-metal fad). So, my list:
Motorhead,
Iron Maiden,
Black Sabbath,
Misfits (not because they are metal but their whole style was borrowed a lot by pretty much every thrash band),
Venom,
Slayer,
Megadeth,
Metallica,
Pantera,
At The Gates.
Then I get confused with nu-metal. I think the important nu-metal bands need a mention, even though now we're way past nu-metal and it's influence is limited at best.
Korn and (questionably) Deftones helped shape nu-metal. I'd say Limp Bizkit but I detest them. Slipknot brought "metal" to the mainstream and as such caused a nu-metal backlash by stupidly claiming they were thrash. Not so.
We now come to the current trend of metal. "Metalcore" or a mix of Iron Maiden-style melody, biting Slayer thrash and brutal Sick of it All-esque hardcore. Defining bands, I feel anyway, would be:
Unearth and Killswitch Engage. I'll add more of these bands when I remember more!