Yes Minister, for me.
You can watch every episode time and again and still not be sure how they're going to end because it's so beautifully scripted and paced.
The best (structured) episode...ever! Page 3
For me it's got one of any number of Fawlty Towers episodes. Though I'd say the one I think is best structured overall is 'Gourmet Night', an episode where everything just falls into place.
Lead Balloon is great for structure and intertwining plot threads, the episode with the spoons is particularly clever.
I'm going to say most of Arrested Development.
It's difficult to see exactly how structure affects the 'performance' of sitcoms.
Maybe someone should write a profound doctoral thesis on the subject.
I might put forward RD's 'Gunmen of the Apocalypse' and 'Back to Reality' episodes as being prime examples of 'play within a play' artfulness which seems to work very well.