KH1 / KH2 are worth playing for sure. MAYBE Chain of Memories if you can deal with a decent but gimmicky battle system and GBA graphics. Re: CoM if you really liked Chain of Memories and want to see it with normal KH graphics.
Neither Days nor BBS were particularly important or entertaining. They're both way too easy, with the combat system feeling a little bit dull compared to KH1/KH2. I kind of liked Days characters but the game's pacing was a bit too slow. Re:Coded was kind of odd. The story was absolutely trivial and the gameplay is really, really disorienting. But at the same time it's the only KH game where you've got 2D platforming and turn-based RPG style gameplay mixed in. If you're bored it's kind of amusing but not exactly great. Can't rank DDD - haven't played it yet.
KH1 / KH 2 > CoM > Re:CoM/Days/BBS > Re:Coded is roughly how I'd rank them.
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