Mask of the Phantasm hands down.
Much as I love the BB series MotP is amazing on every level. They redo his origin without making it monotone and boring (TAKE NOTE MOVIE COMPANIES!!!), they introduce a new love interest and make her the big bad of the show and shaddup about spoilers I rented a copy of this on VHS from Blockbuster when I was a kid, you had plenty of time to watch it by now. The only thing I didn't like was the Joker having a possible identity revealed, even then it's no big deal. I LOVE the ending with the park exploding and her and the Joker just vanishing into the smoke with him laughing the whole time, it blew my mind that they'd do that when I was a kid.
There's a good reason it's my second favorite CBM right after Batman Returns.