This is a package that includes almost all the games in their latest versions.
You should know that the series is known for giving great importance to its Spin-offs, most of which end up being significant in its History.
It is also recommended to play them in the order they were released and not in the order they appear in the collection. The story of KH2 is based on major revelations, which are referred to later in prequels and spin-offs, so if you play the prequels first, you will miss out on these surprises.

1. Kingdom Hearts is the first game. The story has the most twists at the end, the gameplay is very good for its time, and it is interesting to see third parties interacting with Disney classics. Its weaknesses are the platforming, especially in the Tarzan level, and the travel between worlds.

2. Kingdom Hearts: Chain of Memories was initially released for GBA, but it was remade for PS2 and this is the version here. Here you play with cards! The system is addictive once you learn it, but it takes time to get used to. Be careful with the Premium cards, their name makes them sound better, but in reality, you lose them from your deck for battle after one use, and their advantage is that they don't take up much space and are only useful for skills. There is a second scenario, which fortunately takes only a quarter of the time to complete.

3. Kingdom Hearts 2 has much better battles than the first one, it is faster, more user-friendly, and almost no platforming. The story is more complex and many spin-offs are based on it.

4. Kingdom Hearts: 358/2 Days was released for NDS with mediocre reviews. Here it only has the cutscenes as a movie, with many new scenes added for coherence. It takes place between 1 and 2 and during the events of Chain of Memories, but it is best to watch it after 2.

5. Kingdom Hearts: Birth by Sleep was released for PSP. It is a prequel and takes place long before the other games. You control 3 characters and play their stories separately, seeing the story from each one's perspective. If you finish it with all three characters, you keep the saves and unlock the true ending of the game (just make sure to play them all at the same difficulty level for the saves to be compatible with each other). There are some silly mini-games that you can ignore (one of them is for multiplayer, while this version does not have multiplayer).

6. Kingdom Hearts: RE:coded was released for mobile and NDS with bad reviews. Here, fortunately, it only has the cutscenes again, with many new and reworked scenes for coherence.

7. Kingdom Hearts: Dream Drop Distance was released for 3DS and is the last game chronologically before 3. Here, the many spin-offs have started to become tiresome. It has some silly mechanisms that force you to play as another character at regular intervals. You have some Pokémon-like creatures to help you in battle, and with them, you can play mini-games that seem to have been designed for the touch screen and give extra statistics... They are not worth it, and the game doesn't have much end-game content to engage with further.

8. Kingdom Hearts 0.2: Birth by Sleep: Fragmentary Passage is essentially a tech demo to prepare you for KH3. It takes place between KH:BBS and KH1 and is a short story that you won't find in KH3.

9. Finally, Kingdom Hearts χ Back Cover is a video that is like a movie filmed in the engine of KH3. It is based on the online browser and mobile game Kingdom Hearts Unchained χ, and it serves as a prequel to everything, telling the story of the five leaders of the world. Unfortunately, the game is still being released and receiving updates, so the story of the movie does not end.

The package includes 5.5 games and tells a story for 9! It is worth every penny you spend. The downside is that many of them require you to download patches in order to play properly, and they are not all arranged in the order they should be played, so it loses some collectible value because of that.
If you play them thoroughly or play them in Proud mode (not recommended for beginners), almost all of them have an extra ending to prepare you for the next game.