Idk if this is the worst per se but a good example is Russian Doll.

When I see questions like this I'm not just thinking of shows that had a good first season but then a step down in quality. There are lots of shows like that.

I'm thinking of shows where the first season completely stands on its own, perhaps even intended to be a miniseries, but for whatever reason a second season is greenlit.

Russian Doll had a fantastic first season that told its own story. The second season just felt unnecessary and was pretty forgettable.