I haven't seen a lot of the recent cable or Netflix series, so I really can't speak for all time. I basically agree with the Sopranos assessment - a couple of good years.
I remember TV back into the early 60s, and I think the best series I've ever seen is Hill Street Blues. They probably got over 100 episodes before the writing ran out of steam.
Edit: of course, I'm talking dramatic shows here. There's probably a dozen sitcoms that are better than any drama that's been on.
Hill Street Blues took a dive in latter seasons, mostly because most of the original writing staff left, and the new writing staff created characters and plot lines to bump up their residuals (I think they would have received far, far less if they maintained the status quo).
Of course, there wasn't much they could do about Michael Conrad passing away from cancer in Season 4. That was a difficult character to replace.
