Kingdom Hearts is honestly one of the best series I've ever played.
Story-wise, there isn't a single game in the series or spin-off series that I don't like.
Gameplay-wise, there was one game that I can think of that I didn't like much - It was for the Gameboy Advance - something with cards or something.
But easily one of my favourite series ever. And the overall concept is insanely cool.
Square-Enix is a pretty amazing developer. I mean, they developed all (all, to my knowledge) of the Final Fantasy games - And that's one of the most well-known video game series. Even though the series has become pretty terrible with the last few main-series games.