For me it has to be Black Sabbath. The first song off of their first album, released in the Feb 1970: The last minute and a half of the song was just so amazing for it's time....
You can say that all kinds of songs were a pre-curser to metal, but this was the first band that actually WAS metal, IMHO.
(it was also the first band that I saw in concert. I'll never forget it....)
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