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.