Back in the day, these bands were considered to be 'hard rock' and I can't recall the 'heavy metal' term being used much before the mid to late 70's.
But these bands were the ones that got me started, in order...
Led Zeppelin
Deep Purple
Blue Cheer
Black Sabbath
Blue Oyster Cult
Although, of those bands, the 'heavy metal' tag probably only applies to Black Sabbath these days.
So, if that's the case, it would be this band, this album and the song 'Iron Man'