Todd VanDerWerff at South Dakota Dark has finished up his epic series on the 100 best American TV series of all time (plus "sidebar" posts on shows that didn't quite make the list or overrated shows or one-season wonders). It's a big damned thing, a great overview of television and pop-culture history and the role each of these shows played in shaping TV history.
I don't want to argue with Todd about what should or shouldn't be on the list because a) It's his list, b) Most of us will agree with most of the choices. I will add to his statement that "TV is the easiest art form to overrate" that TV is also the medium where the evaluation of a show can change the fastest and the most radically. A book or a movie is a product of its time, and when you look back on it 20 years later, you can see it in the context of its time. But TV shows date almost instantly; a show from 2001 looks, feels and plays differently from a show that was made five years later. TV has to be produced so fast that it depends heavily on the basic production process that's in place -- the process of how things are made, how much money can be spent on certain things, how things get produced -- and as soon as the process changes, TV changes.
A movie, which has more money and time at its disposal, can sort of create its own unique process: Star Wars or 2001 were made differently from any other movie of their respective years, but a sci-fi television series has to use more or less the same production process as any other show. Which means that while Battlestar Galactica is a great show and will undoubtedly hold up, its methods of production will still date very quickly. Twenty years from now people will look at it and clearly see, or at least feel, that it's a product of a time when shows were produced in a certain way, shot in Vancouver instead of wherever shows are being filmed twenty years from now, and used certain kinds of shots and special effects techniques.
What all that means is that more than in any other medium, when you evaluate a TV series you're evaluating two separate things: how it played at the time, and how it plays now. Both are important, and Todd takes both into account ("how it played at the time" shapes the "influence" portion of his evaluations). But the two can be incredibly different; watching an hour-long drama now, whenever "now" is, and then watching it two decades later is like watching two shows. Not just because times change but because context changes; on the original broadcasts you have all kinds of promos and surrounding publicity and chatter that helps shape the way you look at the show. Someday soon we'll return to some of the recent greats -- the Deadwoods, the Sopranoses -- and be shocked to see how different they feel. But the fun of television is in part in the fact that it dates so suddenly and so unexpectedly. The question we ask in revisiting any show is "does it hold up?" which not only means "is it too dated to be entertaining?" but also "how has the passage of time changed the experience?" Sometimes time enhances the experience by revealing things that no one could have been aware of at the time. Sometimes it just makes you aware of the bad special effects and voice-overs that you didn't notice in 1983 or whenever. But that's what makes it interesting, that you don't know what you're going to get. With a book, you can find new things on re-reading. But with a TV show, even a short amount of time can make it different than it was; you're not just finding new things but experiencing a different show, because a 1997 show watched in its original broadcast is not the same as a 1997 show being watched in 2007.
Also, I didn't have time to go through it title-by-title, but it sure seems that a lot of the shows on the top 100 list were snubbed at the Emmys -- especially the dramas, where the "best drama" category has been famously erratic (though The Rockford Files did manage to pull off a surprise win one year).