Led Zeppelin were definitely very important to the development of heavy music. My father always tells me that they used to call Led Zeppelin "heavy metal" back in the days, even though nowadays most people think of them as just classic rock and not all that heavy. Still one of the greatest bands of all time.

Of the 70's heavy rock bands, Black Sabbath, Deep Purple, Motörhead, Judas Priest, Scorpions, Rush, AC/DC and Budgie all have stuff that still holds up in terms of being heavy.