What games are worth playing in this series, assuming I just want to play an action RPG where you run around Disney worlds, as opposed to actually caring about the overarching story?
KH1
KH2
Birth By Sleep
Dream Drop Distance
Those are IMO the most significant titles also filled with new and unique Disney worlds in each.
The Disney worlds are pretty lackluster. They're like tiny little shoeboxes. Play Kingdom Hearts Final Mix and Kingdom Hearts 2 Final Mix. Best bet is probably the PS4 version.
Birth By Sleep >>383558112 is a terrible action game, avoid it. It's like "press the awesome button to watch flashy cutscene attacks where you're invicible over and over" and yes that's also what the other games are like but, uh, BBS just takes it to a whole new level of greasiness.
Play 1 Final Mix, 2 Final Mix. That's all really.
If you like those games then play BBS and Chain of Memories/Re: Chain of Memories