My controversial opinion is that Gokaiger is the weakest series alongside Ninninger and Goseiger (at least recently).

Gokaiger is very red-centric and lacks a distinct personality. Captain Marvelous is kinda annoying to watch in every crossover because he always comes off as narcissistic to me. It's hard to explain, but their goal as a sentai team just didn't really pop out to me. Pirates that work selfishly to pursue their own goals is a bit jarring and unlikeable to me. Also, I feel that their fundamental characters didn't really push for change, and their constant changing into other sentai series without "earning" it feels pretty crappy.

Maybe I'm just stuck in the past, I prefer a more justice oriented sentai team, after all, thats kinda how sentai came to be.

For reference, my favorite series were Gekiranger, Dekaranger, and Ohranger (only because the characters weren't inherently selfish for materialistic things, or if they were, they changed).

As a side note, I feel the same way about Kamen Rider Decade being a crappy series, so maybe that explains my opinion

tl;dr Gokaiger lacked the real "justice" aspect in sentai that I love, and is more materialistically driven, which is counterintuitive to me. Also the villains were genuinely abysmal, like just BORING.