Hm...this is a relatively thought provoking question. Because I am OCD...I *must* expound with copious detail...
Nostalgia ousted from my standpoint, the original Kingdom Hearts to the day still ranks among my favorite in both narrative and gameplay. While the gameplay is "dated", the challenge it presents is irrefutable. In later installments, it grows increasingly difficult to lose. The worlds entailed exploration, forcing you to explore every nook and cranny. The platforming was handled rather well, and it encouraged the player to stay on their toes. Not to mention the occasional puzzle to sever monotony. Aesthetically, I also feel it captures the feel of the original (Disney) source material the best.
From the story, unlike many of the later Kingdom Hearts games, everyone world you visit legitimately holds precedence in the plot. It all interlaces with one another. We see character development in Sora and Riku throughout the road of the story; and are given a great gist of the characters. There is an underlying tension always present; whether it is because the Disney villains (who are actually important in this game) are nefariously plotting against you, Kairi's heart is lost, your best friend betrays you and succumbs to darkness, or you yourself become a heartless. Even the ending was breathtaking.
Chain of Memories, (and conversely Re:Com) whilst engaging, is easily superseded by its successors the the franchise. It suffers from banal level design, and the overall repetition of those aforementioned levels hampers any real immersion. With everything randomized...even something as mundane as collecting treasure chests is disjointed. The combat engine can be considered a downgrade from its predecessor, and constantly having to recharge the cards undoubtedly stifles the gameplay. While sleights are a nice feature, they do very little to liven the combat. That said, I have no true qualms with the battle system of COM. It is just...not traditional Kingdom Hearts per se. It retains elements of the original, but the latter is too divergent.
As far as the plot is concerned, Chain of Memories ranks among the best for me. The characterization is superlatively executed; all of the new antagonists hold memorable traits. The very prospect of Sora losing his memories as he gradually ascends Castle Oblivion, toppled with Namine's role in it was delivered nicely. There was never a dull moment with it (aside from the inane pleasantries which took place during the worlds.)
Kingdom Hearts II held high production values, albeit it was lacking in substances. It introduced a plethora of new mechanics as Drive Forms, Reaction Commands, and a library of abilities which add to Sora's intricacy. The problem is that all of these abilities are extraneous as the core gameplay is oversimplified. They only serve to make an already easy cakewalk even more effortless. Magic is never mandated in combat. Summons hold no genuine purpose and are never compulsory for completion (example, in KH1, I used summons a lot, especially Tinker Bell.) This simplicity of this title negates any necessitation for that. The environments are bland and limpidly uninspired. Each suffers from glaring linearity; effectively holding the player's hand through the entire experience. This is only worsened by the more compact worlds in general being devoid of any nuance of platforming (which was abundantly present in Kingdom Hearts 1)
As far as the narrative goes; it was appalling. Roxas, a new character holding a chasm of potential is introduced, and then haphazardly expunged from the plot until the very end. He served as nothing more than a plot device. Namine, Diz, and Axel shared similar fates. Organization XIII's role vexed me. In Chain of Memories, they were callous and cunning in their tact. They felt like a true threat. Their role in KH2 is miniscule, despite allegedly being "main antagonists." None of the events or characters introduced aside from Axel were even mentioned. In hindsight, they were only threats due to them dispatching Nobodies throughout the worlds. Their purpose was far from evil; in fact I barely consider them "real" enemies. It is exemplary of slipshod writing when we fight Demyx, Xigbar, Luxord, and the game makes no effort to even mention their names. Not once in dialogue. How can they be antagonists if we are ignorant of their names? Throughout the story, Organization XIII did nothing which was perilous to the worlds. Xaldin being a minute exception. They were so poorly devised as antagonist, the only reason Sora traveled to the "The World That Never Was" was because Kairi was (pointlessly) kidnapped. Had this utterly inane kidnapping never transpired-Sora would have no incentive to defeat Organization XIII. Also, do not get me started on how Riku was massively downgraded to a side character. I could continue on, but I digress.
358/2 Days from a gameplay perspective, was almost like playing a slightly degraded version of KH2. However, there was challenge present in Days, and it was therefore possible to die-so this was balanced out rather efficaciously. I have no real gripes about the combat. My grievances aligns with the worlds, redundancy, and bewildering repetition. The missions yielded provided little variety, though this did not irritate me nearly as much as constantly revisiting the same worlds incessantly. I appreciate that Agrabah, Neverland, and Beast's Castle had subtle to major design changes; but that does not mean that I would not grow weary of looking at them. Gameplay, while fun, became a coaxing chore after a while. The pacing was horrid at times. That aside however, platforming and the ever so loved dodge roll have returned. This mitigates most of my complaints to a degree.
Plot wise, this game is tied with COM for its amazing script, and it some areas it is arguably better. While the plot advanced at the pace of a snail at times; when it did come, I was on the edge of my seat. I adored the relationship between Roxas, Axel, and Xion and how it both evolved and deteriorated. The foreshadowing of Birth By Sleep was well done as well. This game by far, is the most emotional for me as far as the ending goes. The ending is by far my favorite. This game teaches us what Roxas truly was like...and not the angsty computer bashing, Keyblade tossing, Diz slashing, screaming kid he was in KH2.
Birth By Sleep indisputably holds the trophy for most intuitive gameplay. The newly introduced combat mechanics (D-Links, Command Styles, Shot-Locks, etc) made for a totally revamped Kingdom Hearts experience. The ability to customize and meld new commands was a wonderful addition. Nearly all of the worlds were new; and not blatant rehashes of per-established material. There was nothing reused in this game; which is why I love it so much. And the fact that platforming and more non linear worlds were reintroduced made my heart exult with joy.
Story wise, while I predicted the ending down to the "T" (almost) I loved the story. While it did not fully live up to my expectations (I expected Terra's allure to the darkness to be more akin to Riku's in nature-and I also anticipated far more distrust and tension between Terra and Aqua) but I digress. It felt extremely similar to Kingdom Hearts 1 in most ways. Do not get me started on the flabbergasting plot twists which crept in. It was...a remarkable experience. And the library of content it hosted ensured that I would be doing many more things Post-Game.
I will omit Re:Coded as...well...I find it completely pointless. Gameplay was a near clone of Birth By Sleep. And the plot...LOLWUT?
That said...my favorite is somewhere between Birth By Sleep and the original Kingdom Hearts.