I currently started the series, only played KH1, KH: CoM and 0.2 BBS for some reason, and i only heard praises about BBS Final Mix combat which i can't wait to play, its really worse than KH1? Because KH is really not that good actually. I even enjoyed CoM more, especially when i figured out how to properly use sleights.
Zeroth place: Chain of Memories/Re:Chain of Memories
I think it is fair to view CoM separately from the rest of the bunch, seeing how fundamentally different it is. It's also the series entry I consider to be the most underrated because of this. CoM is fun, but definitely not for everyone, but if you like the card management and are able to exploit the meta, this isn't so bad. Far better than its word of mouth suggests.
Sixth place: 358/2 Days
Neither of the two DS entries in the series have particularly good gameplay, but the worse of the two is without a shadow of a doubt 358/2 Days. Apart from having a skill system that requires you to spend more time with it rather than gameplay proper, it also controls rather clunkily. Yes, playing as the Organization members is kinda cool, but overall its not good. Also, fuck Leechgrave. Worst boss in the series, period.
Fifth place: Re:Coded
Again... none of the DS KH games are good. Re:Coded is a tad bit better than 358/2 Days by virtue of not having a near-indestructible boss, but other than that it's just kind of... meh, I guess would be the appropriate word. The matrix system is kind of cool in some ways, but again, tinkering around with it is an unpleasant necessity in parts of the game.
Fourth place: Kingdom Hearts/Kingdom Hearts Final Mix
The uncomfortable truth of the series is that its first entry was only ever decent. It's not bad, but it has a lot of design decisions that were not taken into account for the following games for good reason. The controls feel stiff, the menu lacks some key parts from the successor and the level design ranges from OK-ish to confusing af to outright rubbish (looking at you, Atlantica).
Third place: Dream Drop Distance
I was honestly a bit confused over whether I should place this third or Birth by Sleep. Then I remembered BbS not having Pokémon. Deal settled. OK, real talk - I don't hate the dream eaters as much, but I just never bothered with them to the point of having raised the undyingly best team. It also has those freakishly annoying boss fights towards the end of the game. Good god, the last boss was a PAIN.
Second place: Birth by Sleep/Birth by Sleep Final Mix
This is a close second. I think what makes BbS less memorable in terms of gameplay is that it has some stuff that, in my opinion doesn't work as well as it should. It also has some elements that I just couldn't be bothered to tackle - ice cream, for example. Never got all of it. That said, combat itself is fun, varied and builds upon the foundation of Kingdom Hearts II well without dressing it up too much.
First place: Kingdom Hearts II/Kingdom Hearts II Final Mix
So, as I mentioned earlier, KH I was only decent and had problems. The best way to deal with that? Trim off the fatty bits and make the good parts better. More games should do that, ftr. Kingdom Hearts II stripped the first game of its confusing level design and the accompanying adventure parts to streamline the game into a fun, high-energy action RPG and built everything extra around just that. The result is just magical - though it's more rationale than arcane meddling.