Is Deep Purple considered heavy metal? If so Lazy’ on Machine Head is my nominee. Agreeing with the poster above that Deep Purple might have given the genre a kick start.
@baconboy I would classify deep purple as mostly hard rock...
I think it is a combination of 70's Sabbath/Iommi style of riffs and Ian Gillan's vocal style (Example Track: Child In time) that paved the way for a lot of modern metal. I wouldn't classify Ozzy Osbourne as too much of a metal vocalist.
In fact, here's an example of a track from Sabbath's 'Born Again" album, where Gillan was the vocalist....call it Purple Sabbath maybe....
Black Sabbath - Disturbing The Priest
(Don't be the soundguy that played this on the church PA and disturbed the congregation )
https://www.youtube.com/watch?v=3OXLRuu8YQY