This poll covers heavy rock and heavy metal and to me what makes something metal is really subjective, but yeah, even though zep might have been a prototype for metal it imho wasn't metal. This is is why I quit posting on metal boards a long time ago because of all of the arguing, hair splitting, and limited amount of rock music to discuss because in reality "metal" is nothing but a rock subgenre and again very subjective.I personally don't feel Zep IV is any close to metal apart from Black Dog and lost the vibe off Paranoid.
Gonna go with Machine Head.