I always preferred Batman: The Animated Series to Batman: Beyond. However, this is not to say that B:B was a bad show--far from it. It was great, in fact.
Return of the Joker is one of the greatest animated movies of the last decade. The whole thing was intense, memorable, and action-packed. Plus, the dialogue was brilliant. I hope you've seen the uncut version and not the pussified TV version, though, because taking out all the violent parts really numbs it down badly.