i dont really know the answer, but the first use of "heavy metal' was from a book called 'The Soft Machine' in 1962:
"With their diseases and orgasm drugs and their sexless parasite life forms - Heavy Metal People of Uranus wrapped in cool blue mist of vaporized bank notes - And the Insect People of Minraud with metal music"
The 1st use of the term in music was in 1968 by Steppenwolf in 'Born to Be Wild'.
In my opinion the first TRUE metal band would be (as much as I loathe them) Black Sabbath. I have never considered Led Zep to be anything more than hard rock. But if you asked me to think of the first true hard rock band that would have to be Iron Butterfly who started in 1967. Hell now that I think about it, I would catagorize some "Sgt. Peppers" shit to be hard rock and that came out in 1967 too I think.
If you wanna talk about what I think was the first band who was metal, that KNEW they were metal, and staked the 1st claims to being metal, I would have to go with Judas Priest with their 'Sad Wings' album which was like 75 or 76 I think.
Quite frankly I dont care who started it because for the most part I cant stand any of the bands mentioned above 'cept for the Beatles and Priest!
Anywho its still a fun topic.. let the banter continue!
EDIT: upon further review, i just discovered that Alice Cooper formed the band "The Spiders" in 1965, so maybe Alice is the guy.... I dunno.