For hard rock of all time my list might go something like this...
Born to be wild - Steppenwolf
Woman from Tokyo - Deep Purple (I'd have said Smoke on the Water but frankly I like this track better)
Whole lotta love - Led Zep
But it really is highly subjective to personal taste, and everyone's list is going to look different, not to mention that some will want a distinction between "Hard rock" and "heavy metal".
The only way to do it is to compile a list of maybe 500 of the most notable songs in hard rock history and then let several thousand people vote on them, each choosing their top 100. The songs that get the most votes get ranked the best, simple as that. And the one song that gets the most votes is #1.
The question then is, who gets to compile the main list and decide what songs are included?
This guy
JaM1337 has compiled a pretty decent list, at least in my opinion.
Click here and scroll down just a little ways into the comments, he's put up a top 100 list of his own in sections. I could live with that list, although I'd of course make several personal changes if let at it. :D