Some nice, smooth, consistent quality. Better Call Saul was in my opinion pretty consistent. Single season anime is almost always consistent, like My Dress-Up Darling or Terror in Resonance.
Not dragging on longer than the series needs to seems to be a key point.
I, Claudius is perfect television from start to finish but I dunno if it counts since it was a major production with a bunch of the best actors of all time in it.