It isn't possible to give a straight answer to this question. The Kingdom Hearts series is a Japanese RPG type of games, and in this genre an engaging story and emotional attachment to the game characters is crucial. Additionally, this series has a rather complex story with multiple plot points divided into many games and with numerous characters with evolving roles. Of course, the best solution would be to learn the story by playing all of the titles, but for many, it may not be possible - for example, for Xbox owners.
For new players who don't want to play the entire saga, Square Enix has prepared an introduction to the story in the form of short videos showing the most important events, which will be added with the patch available on the release day. In addition, each KH game contains an extensive glossary in which you'll find most information about previous events. What's important, the game also offers a journey to the Disney worlds with beautiful audiovisuals and engaging gameplay with an interesting combat system, thrilling boss fights and a whole range of other activities such as mini-games or secrets. Even a newcomer will get a lot of fun out of this game.
For people who want to explore the Kingdom Hearts story more broadly, we've prepared a table that will allow you to decide whether to invest your time in the game and learn about its contents.
Significance for the plot
Kingdom Hearts Union X
It contains the events that led to the Keyblade War. The first Keyblade Wielders are shown. You can learn about the past of a few side characters. The game is still being updated and its story is not finished yet. The most concise form of it is the Kingdom Hearts X Back Cover movie included in the Kingdom Hearts HD 2.8 Final Chapter Prologue compilation.
Can be skipped
Kingdom Hearts Birth by Sleep Final Mix
Prologue to the events of Kingdom Hearts I. The plot revolves around the Keyblade War, we learn about the fate of three heroes and the main antagonist of the series. Events are directly related to the plot in other KH titles. We get to know the past of several side characters.
Kingdom Hearts I Final Mix
The first published game in the series. We learn about the fate of Sora, the main hero of the saga and his two friends - Riku and Kairi. The beginning of Sora's friendship with his companions, Donald and Goofy. Introduces the concept of "Heartless". The game that is the best introduction to the KH world.
Kingdom Hearts Re:Chain of Memories
A bridge between the end of KH I and the beginning of KH II. Being unfamiliar with this game may lead to confusion in some moments of KH II. First contact with members of Organization XIII. Important for the development of Riku's character. The combat system is significantly different from the other games in the series. You visit the same worlds as in the previous game.
Kingdom Hearts 358/2 Days
A game focusing on the Organization XIII and its members. Introduces the concept of "Nobody". The ending is tied to the beginning of KH II and is essential for understanding it. Introduction of characters associated with Sora. You visit worlds known from both KH I and KH II.
Kingdom Hearts II
The game focuses on Sora's fight with the Organization XIII. It also reveals the plan and motivation of the Organization. Sora, once again separated from his friends from the island, tries to find them.
Kingdom Hearts Re:Coded
The game tells the story about troubles with a diary containing Sora's previous adventures. For the majority of the game, you spend your time in the digital world. The version from Kingdom Hearts HD 2.5 ReMIX contains two secret endings, Signs of What's Next and Destiny, which are directly related to KH 3D.
Can be skipped
Kingdom Hearts 3D: Dream Drop Distance
A prequel of the KH III told from the perspective of Sora. Sora and Riku must pass the exam to prove that they are ready for a fight that awaits them (in KH III). They enter the world of dreams. Important characters from previous games appear.
Kingdom Hearts 0.2: Birth by Sleep - A Fragmentary Passage
The game is not available separately - it's a part of the Kingdom Hearts HD 2.8 Final Chapter Prologue. Direct prequel KH III