I'd say Pellegrini's last season. There was a terrible sense that everyone was just waiting for him to go gracefully, which was compounded by the knowledge that The Man was arriving. And that includes some of the players, by the way. Some of the matches were just playing out time.
It was sad, genuinely, because his first season was nothing short of glorious. It wasn't just the fact that we were champions (and won a League Cup, don't forget), it was that we did it with fantastic style. Don't forget the absolute caning that we gave to the rags, both home and away. On aggregate it was 7-1. Same as in 2011-12, you'll notice.
I always felt really bad about the cock-up that involved people hanging around for about twenty minutes after his last home match (was it in fact more?). Which meant that most of the stadium had cleared off at that point. He deserved better than that.