If we're talking seasons that are part of a franchise and have a reset with each fresh one, I can think of plenty. Digimon, Precure, and Gundam all have some truly atrocious seasons to their names - Tamers, HeartCatch, and Reconguista as one example of each respectively.
But for longer-running franchises...Pokemon and Dragonball Z come to mind. Pokemon has always been all over the place, but the Unova seasons in particular stand out as absolutely awful, with the two worst companions and most unsatisfying league to date. Lots of pointless filler, lots of tournament arc padding, and Ash's worst squad to date. As for Dragonball Z...well, the Garlic Jr. Saga kind of speaks for itself, and so does the Great Saiyaman Saga. I'm also going to say the Universal Survival Saga that closed off Dragonball Super was absolutely shit, it introduced far too many one-note characters and didn't really do anything in terms of worldbuilding or character development the way the others did.
Psycho-Pass 2 was also terrible. I prefer to pretend that never happened.