Which classic sitcom did NOT end its run ranked no. 1?

I wonder if  Arthur C. Nielsen knew that when he started Nielson Media Research back in the 1920s that his name would one day become synonymous with the success or failure of TV shows.

First measuring TV viewership in the 1950-51 season, what is now known as the Nielsen Company has somehow managed to retain its top spot as the preferred audience measurement group despite numerous technological and cultural changes over the decades. One time I got a letter from Nielson asking to fill out a few questions, and you know I felt like an influencer filling that thing out.

Not unlike Nielsen, there are a few shows out there that have dominated American television. Of them, the American sitcom might be the most enduring and successful format of all time. But only a few have lived out the dream — finishing out their runs at the top of the ratings. Which of these iconic sitcoms did NOT achieve this fabled milestone?

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