Have to agree with MeanMan there - no matter what bands you like or what styles you're into, ultimately it can all be traced back to Sabbath. While I'm a colossal fan of Priest, Maiden and a huge amount of others, I reckon Sabbath played the biggest part in the history of metal, chiefly because they were one of the bands who invented
Heavy Metal as we know it.
Even more specifically, doom began with Sabbath, and later on, they sowed the seeds of
Power ("Hole in the Sky") and symphonic (various songs on the album "Sabbath Bloody Sabbath", particularly "Spiral Architect") metal. They also, interestingly enough, were the first band that I know of to use pro-Christian lyrics (see "After Forever").
Also, where would we be without the classic downtuned sound of Tony Iommi's guitar - invented out of necessity following the industrial
Accident that cost
HIM the tips of two fingers.