Fantastic Four in the 60s
The first decade of the Fantastic Four is, of course, essential reading. Stan and Jack's work laid the majority of the groundwork for the series. However, no matter how classic these stories are, a lot of them haven't aged very well. Early tales are clunky, and Kirby had to refine his characters. But to start the character dynamics were great. When you compare it to super-hero books at the time when Fantastic Four burst on the scene in 1961, it was pabulum. What DC Comics was doing at the time were happy-go-lucky super-heroes whose adventures had no consequences, the stories often ended the way they started -- with the status quo restored. Plots were never serious, there was nothing to lose, and the hero always ended up on top 100% of the time. So to create characters who argued, and had real-world feelings (all be it hokey and dated by today's standard), there were interpersonal relationships between characters that actually evolved. From Lee & Kirby's run, you saw the Fantastic Four at their most progressive, something that had not been captured again until John Byrne took the reins. You saw Reed and Sue go from long-time sweethearts to husband and wife, to parents.
But not all the concepts were great, particularly when you again -- note the date of publication-- certain slang, awful gender roles, and even dated concepts wouldn’t stand up today. Particularly the treatment of the Invisible Girl, who was almost always portrayed as a damsel in distress. Although Fantastic Four #11 did a (never repeated) Q&A story that chastised fans for complaining about Sue being "useless" and pointing out the contrary, she was still relegated to the doting 1960s mother figure, too emotional and "feminine" to make any decisions. By today's standards, you read this (particularly since Marvel has moved into being more inclusive to female readers) that sort of writing -- as much as it is a product of its time -- is tough to swallow without at least groaning.
Lee and Kirby also came up with a rogues gallery for the Fantastic Four most of whom have endured the test of time. Doctor Doom, the Puppet Master, Galactus, the Mad Thinker, Diablo, Dragon Man, Annihilus, Blastaar, and so on. No other creators on Fantastic Four have created villains that have been this long lasting ever since.
Then there was the supporting cast, between reviving the Sub-Mariner, to introducing the Silver Surfer, the Black Panther and Wyatt Wingfoot the Fantastic Four used to be a wellspring of new characters.
You also can't mention Lee and Kirby's run on Fantastic Four without at least talking about the Inhumans. As iconic as these characters are, I honestly think this is a low point of the series. Clearly, as Kirby was getting more creative control by this point, and if you know Kirby's body of work you know what I'm talking about. Jack Kirby was a great artist, one of the most iconic and best of his era, but as a writer -- as sacrilegious as this is going to sound -- wasn't that great. (Those of you who are saying "I thought Stan Lee was the writer!" which he was... sorta... Stan came up with the plots, Kirby usually fleshed out the stories and Stan was left to fill in the dialogue) The Inhumans are one of a long line of Kirby's derivative work. It's all the same. Always involving genetics or cosmic space gods. The Eternals, the New Gods, the High Evolutionary's New Men, the Asgardians... Even if you go back to the 1940s and see his work on Hurricane... Kirby had an obsession with god-like characters that are the product of eugenics. It's tired. You read enough Kirby writing and you start to roll your eyes if you actually read it critically enough. He's not a great writer by any means. Thankfully Kirby's other tropes -- reviving wartime characters and inserting a group of teenagers who get in on the action -- were very minimal in this series, unlike his other works where it was almost constant.
By the end of their run, you're almost worn out about stories about the Inhumans, or androids. The end of that entire run was incredibly weak as Lee and Kirby's detonating professional relationship is very apparent.