>>92029907"Start with Batman: Mask of Phantasm".
It gives the backstories of Batman and the Joker before you even get to the main BTAS.
"Batman Beyond: Return of the Joker" gives a good ending to BTAS even if you don't have time to watch Batman Beyond in its full scope. Though at least watch "epilogue" epeisodes of Beyond for League of Shadows, Bane and Freeze.
There are also two Batman/Superman team ups, a Batwoman flick that ties into "The New Batman Adventures", and Sub Zero: a Mr. Freeze-centric movie.