heavy metal has gone through some changes over the years.
When I was a kid in the 70s, - Led Zeppelin,Black Sabbath,Deep Purple were Heavy metal. Basically,anything with a heavy fuzzy riff.
Then came AC/DC,Priest,Iron Maiden,and Motorhead. The hair bands gave it a bad name in the 80s,then the death metal popped up all over and bands like Zep and Purple were considered "Classic Rock".
If you take it back to its roots and call all of these bands heavy metal,then the discussion gets interesting.
Anyway going by my criteria,my favs are:
Led Zeppelin -the Kings -it all started with them
Black Sabbath - Riff master Tony I
Deep Purple - Made in Japan as heavy as it gets
AC/DC - Bonn Scott- the real deal
Judas Priest - Great up until Point of Entry
Iron Maiden - First of the speed metal bands?
Motorhead - no fecking around with these guys