This again proves to be a very easy choice, but I want to dwell on it for awhile because it is so crucially important. Every character must have a motivation for what he or she does, and although the Silver Age writers frequently forgot about positive characterization for their heroes and negative characterization for the villains, they never forgot that either must have some motivation for what they do.
If you look at the supervillains, they usually start with one motivation: to steal lots of money. But if you watch them closely, you'll see that almost all of them start their second and often later appearances with a different motivation: To defeat the superhero who put them in jail.
Looking at it this way does also emphasize the similarities between Lex Luthor and Dr Doom. Both of them hate their nemesis (Superman and Mr Fantastic respectively), at least partially because they blame them irrationally for botched scientific experiments--which left Luthor bald and Dr Doom hideously scarred.
So now let's look at the major DC heroes' motivation during the Silver Age:
Flash: Barry Allen is already a policeman and he had read the Golden Age adventures of the Flash and this inspired him to be a superhero.
Green Lantern: Hal Jordan was a test pilot who was judged worthy to become the new Green Lantern when Abin Sur died.
Atom: Ray Palmer acts as Ivy Town superhero to help his fiancee Jean Loring become a successful defense attorney so they can marry.
Martian Manhunter: Stuck on Earth so he might as well be a cop.
Hawkman: Sent to Earth to learn our police methods.
Now contrast that with Marvel:
FF: Get superpowers and are generally good people to begin with who are willing to take risks.
Iron Man: Gets a terrific defensive costume and he's already a weapons manufacturer so he might as well fight the commies.
X-Men: Save the humans from the evil mutants.
Daredevil: Fight crime while not being Matt Murdock because he promised his dad he wouldn't fight.
Thor: His motivation is never really explained; he's just a noble god.
Spiderman: He didn't stop a robber and the guy ended up killing his uncle Ben.
Whose motivation in there is compelling? Whose is the most believable? Whose motivation has the reader saying, "That's what I'd do, too!"
It seems pretty obvious to me. Spiderman's motivation is personal. It involves a close family member; family motivations are always compelling. It's directly tied to fighting crime in a superhero costume; nobody else can say that.
Daredevil's motivation is close. Certainly his father's death is what inspires him to break up the Fixer's gang. And the Atom has an interesting personal motivation.
But I think overall Peter Parker's motivation is the best in the entire Silver Age; indeed it is matched only by Batman's for personal drama.