Rotten Tomatoes lists Dr Who as the longest running TV series (36 seasons).
The original Doctor Who is twenty-six seasons long. I only watched two seasons of the original before I somewhat lost interest. Also, when there are forty episodes per season it is harder to find the time to catch up on it. Not to mention the fact that some of the episodes were lost and one has to watch still pictures that accompany the audio that actually did somehow survive.
The new Doctor Who is currently on its tenth season and I have seen every episode.