For me personally, October 30, 1977 against the Bills. And I didn't even see the game.
It was just the second year of the Seahawks being an NFL team and me being a Seahawks fan.
I grew up in Maine and it was the 1970s, so the Seahawks game not being on was a pretty common occurrence. I wasn't interested in whatever game my dad was watching, so I was outside screwing around by myself. I don't remember exactly what I was doing, but I remember I was around the front of the garage and the front of the house some of the times my dad came out.
See, every time something happened in the Seahawks game that got a brief highlight shown on the game my dad was watching (that was good for the Seahawks), Dad would come outside and tell me. He came out a lot.
Basically, the Seahawks were having the kind of performance I liked to imagine them having.
The Bills scored first, 3-0. The Seahawks scored the next 49 points. After garbage time, the final result was 56-17.
But before that, the crazy-to-me thing started happening. At halftime, when the Seahawks were up "just" 42-3, other kids from the neighborhood came over to make sure I knew what was happening. That still didn't seem crazy, because these were the kids who saw me every day and frequently played and talked about football with me. The crazy part started the next day.
I can look at the calendar and see that the next day was Halloween, but I don't remember anything about Halloween from that day. The natural subjects of conversation that day would have been costumes, trick-or-treating plans, maybe what kind of candy our parents had bought to give out. Also, since
Star Wars had just come out that May,
Star Wars would have been a frequent topic of discussion. But I remember none of that. The only subject I remember being discussed that day was the Seahawks' dominance in the previous day's game.
It started at the bus stop. For whatever stupid reason, the other kids and I had decided that it was important to be first in line at the bus stop (

kids). I was pretty independent in the morning, so I frequently got out of the house pretty early and walked to the end of the street to wait for the bus before anyone else, but I don't remember ever having to wait there alone very long. The time would have passed quickly anyway, because I used to like to look at the trucks driving by on the "main drag" and try to get them to honk, but that was usually harder early in the morning. By the way, this was a town in Maine with 7,000 year-round residents (about 20,000 summer residents, plus assloads of tourists, with the most common origins being Massholes and
Québécois), so "main drag" is meant to be said dripping with sarcasm. It was one lane each way with very few traffic lights, with a speed limit of 25 where my street came out, jumping to 35 just a hundred yards or so further west on the inland side of town.
Anyway, as soon as other kids started showing up at the bus stop, the subject was the Seahawks. The kids at the bus stop (some from my street, some from houses nearby) wanted to talk about it. The kids on the bus wanted to talk about it. People got on the bus looking for me and came straight to me to talk to me about it. On the playground at school before the first bell, more Seahawks-related talk. Kids wanted to talk to me about it in the classroom, in the hallways, in the weird little gym in the middle of the school that was also where we ate lunch, and on the playground at recess. Kids I barely knew were coming up to me that day to mention the Seahawks game. And keep in mind that none of them had seen the game either. They had just seen highlights or heard about it.
Part of what made that game important to me is that it was the first time I remember the Seahawks totally dominating a game, something I was frequently imagining happening from the beginning. But more than that, what struck me was how other kids responded to it. I was a Seahawks fan and made no secret of it, but I didn't know
everyone in the town knew. I figured my neighbors and maybe a few friends at school knew, but that day, it seemed like
every kid knew and wanted to say something to me about it. In just over a year of me being a Seahawks fan, pretty much every kid in the town within a couple of years of me in age knew about it. I had somehow come to be known as the town's Seahawks fan.
By the way, I didn't get to watch the game, but when the Seahawks did good things, Dad always knew how to find the sports highlights on news shows, plus there were the highlights from Sunday's games at halftime on the Monday-night game. So I saw the highlights more than once.
UPDATED TO ADD: Check out
this 1978 Bills media guide. The cover has a familiar face to Seahawks fans of the '80s, and the bottom of the page marked 74 (it's the 77th page of the 133-page PDF) has a description of the 1977 Bills-Seahawks game from the Bills' perspective. There's more info about the cover model starting on the page marked 8 (it's page 11 of 133 in the PDF).