I don't think there's any show that is universally hated. Someone, somewhere, will always like it. But in terms of shows that people argue about:
- Decade, as mentioned
- Gokaiger. It's like there's no middle ground, you either love it, or haaaaaate it and it's the WORST THING EVER and an insult to the franchise and a total failure at celebrating an anniversary. People blame it for Go-Busters underperforming, for Kyoryuger being chock-full of merch so that its toyline could repeat the success of the Ranger Keys, and complain about its fans being obnoxious to Power Rangers fans.
- Jetman. There are a lot of heated debates over Gai Yuuki and specifically, his death.
- Den-O, because of its getting so many movies and spin-offs, and being a very wacky series that's basically the total opposite of Heisei Rider's beginnings.
- Go-Busters - again a very divisive series. Both its detractors, and its defenders (who take almost anything as an insult to the show) are very vocal
- Kiva and Goseiger both got a lot of hate, but seem to be somewhat better regarded now that it's several years since they finished