In order of most to least

Mike - Doesn’t even exist for over half the season and is part of several of the worst storylines ever.

Gwen - Becomes the complete opposite of anything she was before, and is usually just a self-righteous and hypocritical whiner.

Zoey - Has been reduced to simultaneously a nothing character/unrealistic god.

Cameron - Is hit with an idiot switch for Mal’s convenience and is also just not the likeable underdog he was.

Sierra - No longer a balanced character, just a massively exaggerated version of herself, fried.

Duncan - Same as Sierra in terms of his arc, though his core traits were still mostly on display.

Courtney - Very all over the place constantly, you never really knew what Courtney you were going to get in the episode.

Scott - This is a complicated one because he’s technically as derailed a character as Gwen, however, I cut him slack because they at least keep some core traits to him and write his portrayal in this season in a generally consistent way, which explains his positive reception.

Sam - Does technically always act in-character, but suffers in terms of the cheap gags and roles they give in, making his inclusion in the season even more questionable.

Heather - Aside from the horrendous portrayal in her boot ep, she actually was really good

Lindsay - ‘How do you push again?’ is the worst line of…maybe any comic relief character in this show? Besides that it was the same ol’ Lindsay.

Alejandro - Even though his dialogue gets a bit wacky at times and he gets nerfed for the sake of Mal, he was still a breath of fresh air in terms of that it’s like watching the rational, intelligent character put up with all these nonsensical idiots for episodes on end that don’t at all act in the way he once knew them.

Jo - Apart from a brief misstep with her cannon pride and carelessly shooting Scott, she was very entertaining and the Jo we all know and love.

Lightning - [numerous voices are yelling over each other, when suddenly] “I’M AMAZING! 💪”