>>716183610KH1 - Worth playing, consider mods to get the original textures instead of Final Mix.
Re:Chain of Memories - Worth playing, but some people strongly prefer the GBA version because the card combat worked much better in 2D. The lore is unfortunately important.
KH2 - Worth playing, the rando is interesting but a bit rough to use, looks great.
KH 358/2 Days - The PC version is just cutscenes. It's worth playing on 3DS as the story is the best of the side games, but it controls like ass, runs like ass, and plays like ass. Great game.
Birth By Sleep - This game sucks, fucked up the lore with Xehanort bullshit, and is using C-lister worlds as filler. Important only if you give a deep deep shit about understanding the story, and even then it won't help. Also you have to play all three routes for it to make sense.
Re:Coded - People swear this is the best of the command deck games but that's like saying it's the toenail infection that tastes the best. Skippable, and the PC version is just cutscenes.
Dream Drop Distance - This game sucks, fucked up the lore with time travel AND Xehanort bullshit, and is using C-lister worlds as filler. Important only if you give a deep deep shit about understanding the story, and even then it won't help. For better or worse the two routes switch back and forth, but you can get juked during a boss fight and forced to restart if your timer runs out. At least the TWEWY characters show up.
X Back Cover - Movie important for lore. Doesn't explain shit.
0.2 Birth by Sleep - A two hour tech demo to explain why Mickey doesn't have a shirt in KH1 even though he had one in KH2.
KH3 - Use a mod to weave the Re:Mind abilities into the base game and it's not as abysmal as it used to be. Still bad. Still not worth the wait. Still gonna play KH4 on release because I hate myself.