Lokimok

I just skimmed through the boxset reviews here at DVDTalk & you should probably give them a look. They did a pretty good job of pointing out the best & worst of each set. 2 standouts I'll mention from the 4th volume are "Over The Edge" for its darkness & "Legends of the Dark Knight" for animating segments in specific artist styles (It also has a pretty funny dig at Joel Schumacher). Other than that it will depend on which villains intrigue you. If you like Batman Begins, you might want to see how Ra's al Ghul is handled in the cartoon.