The 60s, I'd have to look up the dates to say for sure, but I think I'd agree with 1967 or so.

The 70s is really tough but I might put my peak a year or two earlier, in order to get things like the BWS Conan and the Severins' Kull in there. OTOH that might leave aout a few things too, like HtD, or Gerber's Defenders.

The 80s, I suppose I'd have to pick 1980 or 1981, when I believe at least MoKF, Miller's DD, and perhaps the Thomas/Pollard Thor/Celestials epic were still going.

The 90s, etc? If I were an evil computer in one of those old tv shows, the hero could make me explode by asking this question: "You ask ... best of 90s Marvel ... but ... there is no good 90s Marvel .... but you ask ... best ... but ... Contradiction! Contradiction! Does not compute! Does not - Cannot compute!" And then the smoke starts coming out of the computer and they're all saved.

Don't really have an opinion on the other decades, but for the 1970s, it's easy: 1979.
The Claremont/Byrne/Austin team was taking X-men to the next level, Michelinie and Layton were doing the same thing for Iron Man, Michelinie and Byrne were doing great work in the Avengers, some guy named Miller started drawing Daredevil, Marvel Team-up and Marvel Two-in-One featured their best multi-part stories ever (amnesiac Black Widow and Project Pegasus respectively) and the Micronauts, a series based on a licensed property based, a mediocre toy line, was absolutely amazing. Near the end of the year another surprisingly good series based on an even less remarkable toy, Rom, was also launched.

1960s: While some would cite the year Marvel jumped into the superhero business, 1966 would be the standout year for the publisher; the Fantastic Four had reached its zenith in redefining the group comic (until The Avengers would have their turn at it in two years), with Lee and Kirby in perfect sync. Then, there was The Amazing Spider-Man: instead of the title stagnating with the end of Ditko's run, Lee continued to evolve the title character into one of the most unique protagonists ever seen in comics, with Peter Parker's maturation mirroring a number of concerns/issues experienced by teens and young adults, hence the reason the character was so obsessively embraced in the youth culture, far more than the other popular teen character of that day, DC's Robin (then enjoying an explosion of media saturation as the so-called "Bat-craze" swept North America). The arrival of John Romita as the title's artist (and occasional co-plotter) was one of the most seismic impacts in the medium's history, as few other artists ever placed such a defining hand on a character to the degree that it quickly became the way all other artist's interpretations would be judged. It is no surprise the title would become Marvel's most successful of the 1960s, overtaking the F.F.'s throne.

In 1966, Spider-Man--with his mix of high adventure, character self-reflection and realistically applied series of Greek tragedies--became a true phenomenon, and as in the case of Lee & Kirby on the F.F., Lee and Romita on ASM was a brilliant creative monolith that made most of the industry take notice, and have tried to reach those heights ever since. Obviously, there were more creative highs for Marvel in 1966 (like he launching of Fantasy Masterpieces, which effectively capitalized on the growing popularity of revived Golden Age characters like Captain America and the Sub-Mariner), but I dare say if the rest of Marvel's 1966 output was lacking, the triumphs of the Fantastic Four and The Amazing Spider-Man still would have shot Marvel to the comic book stratosphere.

1998 was my first full year of reading American comics, so of course I agree 100% with the above

1990s: 1994's Marvels.

By the 1990s, Marvel had published numerous in-universe histories (straight or alternate), but few ever captured the total essence of the elements and emotions that made Marvel's creations part of the modern day pop-cultural mythology in the way of this series. To say the "you are there" / Man on the Street perspective of the dawn and growth of superheroes in a real world--how they would really be viewed was powerful would be a gargantuan understatement. On that note, and quite frankly, this series presented the mix of real world and fantasy heads and shoulders above the bloated Marvel Cinematic Universe of today. All of those dollars, actors and marketing thrown at Marvel concepts, and about 95% of them paled in comparison to the way the heart of Marvel was presented in Marvels.

Of course, in the art department, there were many painted covers (and a few interiors) in comics long before this series, with the work of Neal Adams, John Romita, Ken Barr, Bob Larkin, Alex Niño, Earl Norem and others who took the basic comic character to new heights on Marvel magazine and TPB covers, but Alex Ross' sense of making heroes appear as one would imagine among familiar settings was revolutionary. The characters were not just painted, but a breath and energy was put into every character--from hero to random person in the crowd, which added to Busiek's engaging journey through three "Ages" of Marvel history.

Marvel's eXtreme failings in the Liefeld/Lee/McFarlane/Larsen/all things mutant (and all influenced by those markers) between the late 80s and into the 1990s had the company turned into a noisy mess--a fading shadow of its stellar past. Marvel (largely) in name only, but this one miniseries--both quietly patient and seismic at the same time--reminded the world of what Marvel had been. Unfortunately, Marvel never reached this peak ever again.