If you aren't sure which final fantasy to buy, buy 6,7,9 and 10. Read the premise for each one, or pick whichever one looks best to you to play first it won't matter which. You will then understand everything.
I genuinely hate how the 2 beat combat games of all time are agreed to be DMC5 and KH2 and no other game Dev thinks it's a good idea to have air combat and combos.
Mitsunori Takahashi was the lead battle planner who designed Kingdom Hearts II’s combat. He’s credited with its signature fluid combos, Reaction Commands, Drive Forms, and Revenge Value system.
He later contributed to battle design on Final Fantasy VII Remake. No major public credits on his current project have surfaced recently.