First season was a nice surprise, and was great, maybe one or two not as great episodes but it was a good season.
Second season had good episodes and some meh ones but a strong finale.
Third season makes the S2 finale meaningless and has some of the worst or most boring episodes, only one episode felt like a good episode from season 2, but the charm and surprise from season 1 is completely gone.
It's obvious the more popular the show got, the more the writing got shittier, and it all falls on Dan Harmon. Whether it's because of the inclusion of female writers, Dan Harmon's divorce, an inflated ego on the show's popularity, or he just got lucky making a decent first season that took everyone off guard, but went tumbling down after the second and third once people were paying attention.
Also Doc and Mharty is still the best episode.