19. Kingdom Hearts: Chain Of Memories
Chain of Memories looks like a weird card game spin-off but nope, if you don't play this, Kingdom Hearts 2 is going to be really confusing.
Of course with the existence of Kingdom Hearts: Re:Chain of Memories, a remake of this title for modern platforms you don't need to take your GBA out to experience this but still make sure you experience it in some capacity, not just for the story but also because the game's actually really good.
The card gameplay here is the most strategic the series has ever gotten, it's fun building up your deck and trying out different strategies against your enemies.
It isn't perfect though. The biggest problem is that the story is very similar to the original game as you spend Kingdom Hearts: Chain Of Memories in Sora's memories of that game.
If you want to play through the series you better learn to love the worlds from the first entry. This isn't even the only time that this is pulled, Kingdom Hearts Re:Coded does the exact same thing. It's time to move on Tetsuya Nomura.