The greatest Television show is similar o the greatest Film, there is no such thing but the idea exists in the minds of the public. What is considered the 'best' is based purely on ones imagination while the best-made is a different story as it counts upon the production of the piece of cinema.
There are those who would consider common modern trends such as The Hunger Games, Twilight, Harry Potter, The Walking Dead or Divergent to be incredible pieces of cinema and while they are crafted to suit separate audiences there are also classics that a lot of us from the older generations consider to be far greater such as Shawshank Redemption, The Green Mile, Forrest Gump or hell, even the original Star Wars Trilogy.
Best is a matter of opinion, the question should be "What is your favourite?"