Babylon 5 has had a great story arc of five years. Problem was that the arc had to be cut short immediately after the first season, when one of the main characters decided to pursue other opportunities. Fortunately, this all worked out great and it's still one of the best shows ever to me.

Lots of things happened during the show, with characters that still stand out in my mind. Human characters, but most of all the alien characters. Ambassadors that are enemies / friends / enemies. Political intrigue. Beautiful ships. Large scale genocide where you actually care for the dead. Undercover, fascist organizations on earth. A dash of humor in the right places. Human telepaths with a credible (and immensely scary) backstory.
And even angels and demons, or at least those that presented themselves on earth as such since the beginning of the 20th century...

As soon as you figure something out, the rule is that it's likely different from what you just figured out.

Not so great was the ending of season four. Or rather, what happened after it. B5 had been on the line to get cancelled for almost every season, but the go-ahead for season 5 was so late, they were sure they were going to get cancelled after 4. So, JMS (the show's creator) decided to put all the story in season 4. Action packed, lots of things happening, ups and downs, loved it.

Then... season 5 was green lighted.

And the bottle with "story" in it was empty. JMS did try to make it into a full season (he had some ideas for a rebellion on a distant world which would be made into a movie I think, or otherwise books, but he pulled it all forward and put it in season 5), but really, the story was a bit thin everywhere. I liked it overall, but not as immensely as the seasons before. Partly because of another unexpected character change mid-season, which, well, did ... not work out so well.

The biggest bad thing however, where the spin offs. Almost none of the movies where any good (at most "enjoyable" for "River of Souls", to "horrific" for the spin-off series "Crusade" ). The books, those that were published, were actually kind of good. I enjoyed almost all of them immensely.

I don't know what happened. JMS seemed to have sold his sould to write that one five year story arc, but didn't have anything to sell afterwards...