I like to sort of try and be more objective when I rank things, I like to do a "Best" ranking and then a "My Favorite" ranking. Obviously it's not truly objective, but I think there are certain elements of game design you can point to in each Kingdom Hearts game that might make them better or worse.
In terms of favorites, for me it goes:
358/2 Days - Love this game to pieces. I think it gets a bad rap mostly because people try and binge it like you can for every other KH game, and it gets a bit repetitive if you do it that way. Yes, there are some frustrating bosses but... overall? My favorite selection of Disney worlds, I LOVE the panel system, a huge amount of content, and easily the best written story in the franchise. is it the best story? I think so, but other might disagree heavily on that point. Love it.
Kingdom Hearts 2 - Wonderful worlds, wonderful characters, great bad guys... I love this game to pieces. 'Nuff said
Kingdom Hearts: Chain of Memories - I really enjoy the classic Chain of Memories. It's a gorgeous sprite-based game, one of the best looking and sounding on the GBA, has an engaging story that introduces some of the series' best characters and music, the card system is unique, challenging, and fun to play around with. I love it all... well, most of it.
Kingdom Hearts 1 - Lower than I think most people would put it, but I think KH1 suffers from nostalgia blindness sometimes. Some things are stiff to control like jumping, the remake makes some things harder due to framerate and control issues (which is not a good difficulty spike) some of the worlds are terribly designed and confusing, but I mean... it's got a fun if simple story, some great bosses and fun worlds to explore, and is still a great game in it's own right.
Kingdom Hearts: Birth By Sleep - This game... I'm so torn on it. On one hand, some of the story elements are really moving and bring me to tears, and it's awesome to see where things started. On the other hand, the gameplay is just not fun at times, when spamming certain moves is the best way (or in some secret bosses cases, the only way) to victory. Not nearly as bad as DDD but... ya know. The dialogue can be clunky, and the worlds are big and empty for no reason, but I still like it a lot.
Kingdom Hearts: Dream Drop Distance - Plenty of flaws: time travel, magic spamming, the dream eaters, Disney world storylines... but It's still a really fun game. I enjoy flowmotion in this game way more than in KH3, and the dream eater system, while frustrating, adds a lot of nuance to combat. The bosses near then end portion of this game are tremendously fun as well, so that bumps it up for me.
Kingdom Hearts 3 - I was immensely disappointed with various aspects of this game. Lack of Disney world integration, the Kairi fiasco, full of sequel bait, overly easy, and worst of all, it was a very frustrating end to the series. I wanted so much more time with the characters, but instead we got a rushed maze and a nonsensical cliffhanger ending. There's more to say but, eh.
Kingdom Hearts: Re:coded - This game gets a bad rap. Sure, it's plot is kinda stupid, especially the new stuff near the end, but honestly? The gameplay is really good, if a little repetitive at times, the system for leveling is fun, Olympus Coliseum is a blast (for a little while before the grind) and I enjoy the oft weird, and unique boss fights. I've come around on it over the years.
Kingdom Hearts 0.2 - Is this even really considered a game? It's a demo, at best, and nothing that happens in the story was relevant. Aqua controls poorly, there's a lot of issues with gameplay... but the environments are really sweet, so there's that.
Kingdom Hearts: Re:COM - I do not know why I dislike this game so much, but I really hate it. It took all the charm and uniqueness of COM and ruined it on the PS2. While I love the cg cutscenes, everything else feels like a failure to me. The rooms are too big, the combat is slow, clunky, and hampered by 3d space. Deck building isn't a good as it used to be due to card changes, and even the stuff they added in makes the game a bore. It might be just me, but this is one of only two KH games I consider "bad"
Kingdom Hearts: Union Cross whatever - I play a few gacha mobile games, and this is easily the worst one. Terrible gameplay, a flawed gacha system, exploitation of players, and a lack of real competition between the unions. Not to mention that the story is edging on nonsensical and is absolutely impossible to fully understand without someone on the forums or the wiki explaining on the original browser game was it's own world and how the dandelions jumped worldlines into a new game on the phone, and then jumped again and yada yada yada. Not to mention that the story comes at the slowest pace of any game I've every played. Easily the worst game in the franchise.
Wow. That was a lot.
In terms of best, I'll just list without reasons, but if anyone cares I can explain it later.
KH2
COM
KH1
DAYS
RE:CODED, DDD (pretty much the same, qualitywise, with flaws in different areas)
BBS
0.2
RE:COM
Mobile games
I think a lot of people rank COM and Days, and Recoded really low because of three reasons: a) they didn't play them until after playing through most of the rest of the franchise/many years after their release and so found them hard to get into 2) simply never played them for whatever reason and have bandwagoned on the OG hate train III) genuinely didn't like the game for some reason
I think people in the third category are in the minority however.