A defining run is simply one that defines the character. A defining run is then one that is used as inspiration by later writers and which established or explored the limits within which a character works well. It is rarely a later run. Now, Silver Age heroes weren't all defined in the sixties.
Beast was defined by his 70s series which established his continual mutation.
Captain Marvel and Him/Warlock were defined by Jim Starlin in the 70s even though they were created in the 60s.
The (first) Guardians of the Galaxy were defined by Gerber.
She-Hulk wasn't defined in her first series. Well, she was, as a monster, but that didn't stick. The one that stick is her Stern Avengers + Byrne FF appearances. She became a legit superhero rather than a menace, and fun loving and gorgeous rather than monstrous.
Captain America was defined by Lee's work from 1964-1971. During that period, he became the out of time soldier, he was a team member of the Avengers in epic adventures, worked for SHIELD as a one-man commando in spy/action thrillers and was engaged in social commentary by his association with the Falcon and his road trip through America.
Little much has been added since. Englehart's run fall in the social commentary model. Brubaker in the spy-thriller one.
As excellent as it was, I can't see how Simonson's run on Thor could be defining. The mythology, soap opera and space opera epics were already there in Lee-Kirby's run.
Wolverine's limited series didn't define him because Wolverine isn't continually immersed in the Japanese culture.
For Daredevil, it's tricky. Miller clearly influenced generations of writers but on the other hand the character has characteristics that Miller didn't touch and that other writers, like Kesel and Waid have touched.
I'd say Wally Wood had the first defining run (he created the costume) but it's subsumed under Lee. Wolfman created the modern DD with more hard edged stories, which Shooter, McKenzie and Miller followed. Then Miller redefined the character by adding elements.
A famous comic book writer once said that he considers Miller's DD an "Earth 2 version."
Mazzuchelli featured some Gene Colan influence. In that his approach wasn't the complete break that Miller was, his run is highly regarded.
The first writer who has combined all the influences is Mark Waid.
I'd say this character didn't have one defininig runs but several. One interesting aspect is how the character changes when you change the romantic element: Karen Page, the Black Widow in San Francisco, Heather Glenn & Elektra, Typhoid Mary & Karen Page, Milla Donovan, Kirsten McDuffie.