Well, Game of Thrones hasn’t derailed yet, but it’s got an edge over most TV shows in that the plot’s already written quite a few seasons in advance whereas most TV writers have to wing it week to week with at best a rough outline of where this particular season is supposed to go and end, if that.
GoT is really more like a series of movies based on a book than a TV show.
I thought that The Shield was consistently strong through its seven seasons.
One could make a case that **ER **was quite good until the later seasons. There were a total of 15 seasons and I stopped watching around season 10.