1. Cecile Horton
I hate that we have to put Cecile at the top of this list but honestly you won't find a more polarizing character in The Flash's nine-season. I really liked her during her recurring appearances on the first four seasons of the show, and I even appreciated how her psychic abilities made her an asset in Team Flash's battle against Clifford DeVoe. But (almost) everything just went downhill from there, and it would be ludicrous to overlook the negative response to the character in the final few seasons. Unfortunately, the creative team did just that and continued to push her to the forefront of The Flash at every opportunity.
What was it about Cecile that made fans angry? It's hard to pinpoint just one thing. I think it definitely has a lot to do with overexposure. She stopped taking things seriously when she was made a full-time character, often finding herself in silly storylines with silly one-liners designed for nothing other than comic relief. While it certainly worked from time-to-time, it was just too much - and a huge backwards step for the once-serious DA of Central City.
The creative team were missing the point of why audiences weren't gelling with Cecile, giving her more and more instead of streamlining, explaining and making sense of her strengths. She was a great friend and confidante to Barry whenever he needed to speak to someone, but instead of developing relationships like that, the show decided to feed into the reasons fans were struggling with the character. Her powers were largely the problem here too, as she would publicly announce what other Team Flash members were feeling in front of other people, which is just plain wrong.
Fans also understandably hated the fact that the final season began to brand her as "the most powerful metahuman"... in a show called The Flash about The Flash, who led Team Flash. Not only was it disrespectful to the legacy of the character at the forefront of the show, it once again placed far too much investment in Cecile when the fans were growing weary of it doing that. And then, the icing on the cake, in that same final season when she officially became a superhero, the final ever episode revealed her empathic powers could... make her fly?
Cecile definitely had a place on the show and Danielle Nicolet did a great job in the role, but the back half of The Flash did the character a major disservice by giving her far too much focus in all the wrong areas. The serious, hard-hitting, and compassionate DA Cecile was so much better than the caricature she became.