I own B:TAS volumes 1 and 2, and I like most of the episodes. Everything with Mark Hamil is great. I'm also a big Harley fan, and episodes with her are always entertaining. I plan to get volume's 3 and 4 eventually, but they're pretty expensive. I can't really even say which one if my favorite, I like so many of them.
There is one episode I'm conflicted about. I didn't like Zatanna not being a real magician in the episode "Zatanna". I get that Batman at that point hadn't really dealt with stuff like that, but the show did end up having a guy who is immortal because of magic pits, so I don't see why Zatanna had to be just a stage magician. Of course, they never really say she isn't a real magician, or atleast there is never a scene that proves she isn't. Maybe she just hid her powers from Batman. It would mess up why Batman would be learning tricks from a real magician and his daughter, but if the DCAU is supposed to be in continuity with each other (Superman, Batman, JL/JLU, Batman beyond all being connected) then that episode of batman would contradict the JLU episodes she's in where she has real magic powers. I wish she had made more full appearances besides just one speaking cameo in JLU, I like her character and it would have been cool to see her get a JLU episode that focused on her.