Kingdom Hearts 1 was easier to me. The combat was alot slower As well as the movement. Kingdom Hearts 2 on Critical mode was the hardest in my opinion. The enemies seem to do more damage and are faster.
I think Kingdom Hearts 3 got a bad wrap. Even though Kingdom Hearts 2 is the best in the series in my opinion. KH3 was great. Fun gameplay, the worlds effected the over arching plot. And it left an opening for the next saga of the series.