>>719687324
KH is for sure. KH2 is a great action game but is where the story starts to get wacky as fuck. KH3 will make no sense unless you either play most of the side content (Chain of Memories, Birth By Sleep, and Dream Drop Distance at the absolute minimum) or watch a video or something to get you up to speed. I think the Final Mix stuff or something has a "story thus far" thing for 3 built into it.
Play 1, play 2 if you liked it and want a generally improved combat game with a slightly more convoluted story, and 3 if you still want more with good payoff. BBS is also worth it, and one of the best side games that also plays similarly to the mainline.