As for JL vs. JLU, it's a bit of a toss-up really.
The first season of JL was definitely uneven, they were still trying to balance action and the characters and having everything make sense in the DCAU.
The second season, though, was fantastic. I think they really hit it out of the park. But if you like huge amounts of characters, JLU is probably best, and I enjoyed it more. I just wished they carried over the hour long format from the JL show, I think it would have helped some of the plots. But then again, you can't go wrong with Dr. Cox riding around in Wonder Woman's cleavage.
And really, any of the B:TAS seasons are good to pick up. Volume 4 might be a bit "kiddier" in look, but it also contained one of the darkest episodes in the whole Batman run: "Over the Edge"