I've been trying to figure at points the show started going downhill a bit and I've been flip-flopping in my head between season 11 / 12, but never really been able to decide and I've finally realised now; the start of season 11 is quite bad, especially the Olivia / Harold thing being pointless. Of course, they kind of pick up towards the end of the season, especially when Anne comes back and starts her scheming, which makes season 12 start off rather strong with the addition of Claudia. But then it sort of starts going downhill slowly, basically starting with Val pointlessly hitting her head and going cuckoo (would it have hurt anyone to just backburner Val / Gary for a few episodes instead of giving them a new problem?).
So essentially, the show hit a sweet spot at the end of season 11 and the start of season 12 that makes up for about a season of episodes, but it didn't last.
The ratings started to get soft in the back half of season 12, even though there is much still there that I enjoyed (Mack and Jason, Anne and Nick, the whole Claudia/Steve thing). But it is true that an early 1991 episode suddenly become the least watched episode of the show up to that point, stripping the title from an episode unwisely aired on Christmas Eve in 1981.
What is notable about season 12 is how it is a relatively low key season after Danny drowns and Jeff is arrested, its as though they wanted the audience to have a breather. Brain injury aside, I like the year because it is long on good character writing.
However, what really killed the show was the brief Romano reign, which had some interesting visual ideas, but had such muddled writing that felt atypical of the characters we had been with for so long, and by the time that team righted the ship, it was too late for them and for the series (the ratings evaporated). I wonder sometimes if the show would have run an extra year if Ann Marcus had been given the reins at the start of Season 13. At least the final season was good.
I do think that, slackening in the second half of season 9 aside (likely due to the writers strike), the Latham/Lechowick era is very underrated.