IIRC, VH-1 ran the 100 greatest hard rock/heavy metal bands and led Zeppelin was the winner with Black Sabbath no. 2. Van Halen and Hendrix were also top 10.
Its sad that many of the groups listed above have been mostly forgotten and are no longer played on the radio (at least not here). I was raised in Chicago and used to listen to the greatest radio station ever called Triad (mid-70's), which was pretty much an underground type station. You heard stuff on there that was never heard elsewhere and was only broadcasting at night.
Here's a few more favorites:
April Wine
Captain Beyond
Alan Parsons Project (not actually hard rock, but close in some respect)
Rory Gallagher (sp?)
