The music, the music, the music, the music, and the music. That's the biggest reason to play a KH game, sure the games are fun but hot diggity damn is Yoko Shimomura an amazing composer. Battle system is simple, but hack and slashes are almost always enjoyable stress reliefs. When it comes to my favorite of the series, it'd hands down be BBS because of the 100% waifu material that is Aqua.