Probably Roberto's first season. It was the only one where we were consistently good since, arguably, we won the championship in 1987. The day we beat Arsenal 3-0 seemed to be the day we had truly arrived. And then we followed up with a preposterously stupid home capitulation to Crystal Palace, with John Stones proving himself an indulgent liability.
We've had the odd moment since then, of course, but the last time we had a continued run of feelgood form was Roberto's first season. Deulofeu, Lukaku, Barkley all playing well. Mirallas not being a self-entitled imbecile. It couldn't last, of course.
Prior to that, you'd have to say the year we qualified for the CL play-offs. We were great at grinding out tight wins that season - even if the football was agricultural.
Joe Royle had a good 18 months as well.
Prior to that, you've got to go back to Howard's glory years.
Generally, we've been nondescript, mediocre, or downright atrocious.