Okay I admit I was so hooked I bought the series on dvd.The story line started getting a bit hokey after the 3rd season after which it was overly violent to the end of the series. I began to lose interest when they started killing off main characters who were the only ones that put hope in the situation and replaced them with evil people intent on control and power. The main character Clarke turned out to be the most controlling and violent of them all. They never did "do better" as we had hoped for.
I was left wondering who does Earth belong to? Who had the most rights to it?
Would it be the grounders who stayed and survived during the apocalypse
The ones who went underground or
The ones that escaped to space? Since they were all from Earth.
All in all I'd have to say the first 3 seasons carried the best story lines.