ledger joker is overrated. not saying it wasn't good. it was great but it's really not a good interpretation of the joker as far as comic accuracy
Which series? You realize there are about ten different interpretations of the Batman comic. Authentic "Joker" is very subjective. Nolan's version carries many of the known comic book themes attributed to the Joker. Killing three prominent officials, multiple stories about how he came to be, and/or the maniacal way he plots.
The best part of the Batman series is that it is open to interpretation. I appreciated Nolan's realist approach to it and ledgers twist. I also liked Nicholson/Burton as well. Different people, different twists. Loved them both.