What are some examples of series that only have a good first season, and should be dropped after that?

The ending of season 1 was perfect. Anything after that is an excuse for nudity and gore.

Agreed. I like the following seasons okay but the first season is nearly perfect on its own.

Season 2 was rough, but I think season 3 was a good soft reboot. After that it's all downhill again, but it almost righted itself.

Kiksuya.