Last one. I was going to reflexively say, "The '80s!" because there are quite a few things from that decade that formed my opinions on certain franchises. With the X-Men, coming in during a Secret Wars 2 tie-in, the issue after depowered Storm wrested away control of the team, it was a disinterest in Cyclops (actual distaste came later, with the '90s cartoon) and generally the rest of the Original 5 X-Men, an acceptance of Magneto as a good guy, fondness for Wolverine's brown costume, stuff like that. With Spider-Man, it was the importance of Mary Jane Watson (and to a lesser extent Felicia Hardy), rather than Gwen Stacy. Also, Norman Osborn was at best a specter hanging over Harry, but the Hobgoblin was the immediate threat.
But, I'd imagine the early '80s, which I'm less familiar with, aren't quite the same. So I'd say, if I were going to pick a 10-year stretch, simply to be arbitrary, let's go '84-'93. That still incorporates the things I mentioned above, but also a lot of other things. Like the New Warriors, or Acts of Vengeance, both things I love dearly, the former having made me a lifelong fan of Mark Bagley and Fabian Nicieza. It includes more of Harry Osborn's sad descent, but also that stretch where Spidey teamed up with several of his old foes who had reformed (mostly when Gerry Conway was writing). The idea that Spider-Man could encourage or inspire people to try a different route is pretty important to my idea of Spidey. I could throw in Doc Ock wearing suits, rather than green spandex, or Venom. I know, I don't like Venom, but it was his overuse in the comics of that time which cemented that. It's formative, just not in a positive way.
You could include things like my fondness for the early West Coast Avengers, like Hawkeye, Tigra, Hank Pym with "scientist adventurer" shtick, Iron Man in red-and-silver armor. I came to them years after the fact, but this was the era for both Rom: Spaceknight, and the Mantlo/Mignola Rocket Raccoon mini-series.
On the DC side, it puts us a couple of years into the Tim Drake era, which also brings Spoiler into the fold. Plus, we had Ray Terrill as the Ray by then, and he's still my favorite DC character, 20 years later. Again, I got to the party late, but Suicide Squad as a title I deeply enjoy. Though not from DC, GrimJack would be another late '80s series I found much later.
In a more general way, I think the comics of the time influence a lot about my expectations about superhero comics. I was listening to one of the Rachel and Miles X-Plain the X-Men podcasts, the one about Storm and Forge fighting Dire Wraiths, and Rachel commented that there were just too many plotlines all at once. Between Storm losing her powers, confronting Forge, the start of the arc against the Trickster, Rogue struggling with Carol's memories, Colossus learning Illyana learned from Belasco, on and on. I saw her point, but for me, that's normal. That's how it's supposed to be. Lots of different arcs, some that go on for long periods of time while others wrap up sooner. Different ones rise to prominence and fade over time. The era I was introduced to comics in is probably why I'm comfortable, fond even, of thought balloons, and obvious recapping/expository dialogue. On a less positive note, it's also probably why I tend to not pay as much attention to an artist's stylistic flourishes, because I kind of take the approach their job is to help tell the story in a clear manner, full stop. I don't think that's wrong, but it devalues the penciler, making them more an appendage of the writer than a partner. Like I said, not every formative aspect was a positive one.
Well, that's it for this series.