I started considering this thread late last night after finally finishing Steranko's run on Nick Fury: Agent of Shield and comparing it to Lee/Kirby's. I think it would be interesting if we made lists of characters that we consider to be definitive, or mainstay classics of the medium, and choose the run for each of these characters that we consider to be "definitive." For example, if you believe that Prince Namor is a timeless classic of the comic book medium, what would you consider to be the definitive Prince Namor run?
This thread is intended to be an ongoing discussion; not a collection of immutable lists, so feel free to add more characters and make changes as often as you like. I'm very curious to see what others come up with, and look forward to pulling many reading recommendations from this discussion.
I'm in a Marvel mood tonight, so I'll start there:
Doctor Strange -- The Mordo/Dormammu Team-Up (Lee/Ditko) Strange Tales #130-141
For five issues before Nick Fury: Agent of Shield began as a second feature and one issue after it's first and most legendary storyline concluded, this extensive epic blazed across an entire year of the Strange Tales title. Here, Marvel's potentially most powerful hero came face to face with an enemy threat that he had no chance of defeating. Instead, we spent issue after devastating issue watching Strange flee in shadows and disguises, desperately chasing every slim hope while watching all of his support structures (The Ancient One, Wang, his Sanctum Sanctorium, and even his public identity) fade from view. These were powerful, dire, and thoroughly humane stories for the sorcerer who we believed could do anything. In the epic style of Tolkein's Lord of the Rings, persistence and bravery in the face of uncompromising defeat lead Strange through fascinating realms of the imagination to the victory that no one but he would have believed was possible.
Fantastic Four -- The Silver Surfer Era (Lee/Kirby) Fantastic Four #48-77
As far as I'm concerned, the Fantastic Four team was never all that compelling, itself. The characters were very likable, but the premise never really won me over. Instead, what I found enticing about the title was how it served as the cosmic backbone to the Marvel Universe, introducing so many of Kirby's fantastic far out space concepts like Galactus, the Silver Surfer, The Watcher, the Kree and the Skrulls. So many of those concepts were first introduced and developed within a two year span, mostly consisting of the Silver Surfer (some of my favorite SS stories of all), but also introducing Galactus, the Kree (which, in turn, laid the groundwork for Captain Marvel), and even Black Panther. This run feels like a Marvel Golden Age to me, introducing and developing an amazing body of work and concepts in such a short span of time.
Nick Fury: Agent of SHIELD -- The First HYDRA War (Lee/Kirby) Strange Tales #135-140
A lot of amazing things happened to SHIELD in the pages of Strange Tales, but none felt as original or captivating as the first major story arc. Here, in addition to amazing gadgets and a compelling cast of supporting characters, Lee and Kirby gave us a clever and menacing secret army with a corporate face. Amidst all the action, character, and high tech gadgetry, a fascinating story was taking place with a mysterious and somewhat scary enigmatic villain at the center of it all. The final revelation of who the Supreme HYDRA was, as well as what finally happened to him, seemed both brilliant and fully worthy of the five issue build-up. Sure, Steranko's art and expanded cast of characters in the later issues trigger fond and fantastic memories for me, but this was the definitive SHIELD adventure, as far as I'm concerned.
Spider-Man -- The Green Goblin Era (Lee/Romita) Amazing Spider-Man #39-122
Though I've never been a huge Spider-Man fan, I am always positively awed by this run. At their best, Spider-Man stories are about the human hero who doesn't always win. Green Goblin always brought that out best in Spider-Man, catching him off guard, unmasking him, complicating things by revealing himself to be someone in Spidey's personal life, creating a situation in which Spidey fails to save the life of his girlfriend, and finally taking Spidey to the point of blind fury before his climactic demise. In my mind, these are the only Spidey stories that are truly worth reading.
X-Men -- The First Claremont Era (Claremont, Cochrane, Byrne, etc) mid 1970s to mid 1980s
Compelling characters, fascinating stories, superb art, and an incredibly tight, ever-developing continuity all define this amazing era that began with Len Wein's resurrection of the team, achieved momentum with the Dark Phoenix Saga, and remained strong into the early 1980s, spanning out into countless one-shots, mini-series, and spin-off titles, most of which were actually very good. Claremont's work on the early New Mutants, Wolverine, and even the Magik mini-series helped to turn the X-Men title into a franchised universe flooding with compelling characters and back-stories. To this day, I truly believe that the X-Franchise during this time was a new Marvel Age all on its own, as well as my all-time favorite output from the company.
EDIT: There are many other Marvel characters that I consider definitive, but I haven't read enough of their stories to be able to call any "definitive" at this point. Some of these characters are:
The Incredible Hulk