Spider-Man? For me Superman is way up there and Spidey is way down there. I don't see that the webslinger comes anywhere near to approaching the greatness of Superman--not even just a little bit.
No argument there, really. Superman is Elvis Presley, the original, the biggest and also the prototype for everything that came after.
I came at this from the angle of which one would I read and re-read? I have the Famous 1st of Action No. 1, but I never read it. I re-read the Spider-Man comics, though, starting with AF #15. It is still so readable, still such a great little story, with that quirky (and totally fitting) artwork that Lee hates.
As an investment, which is not what I was thinking of, it would have to be Action, hands down. Action is the most important comic book, historically.