Jack Nicholson is the epitome of the crowd pleasing, enjoyable and irresistible to watch villain, who, no matter how funny and anjoyable he is, he is still unpredictable, lethal and one to hate (remember he burnt the face of his own girlfriend and killed off his own henchman). No matter how great Heath Ledger's performance is, and how this new Joker feels more like the chaotic and mysterious clown of the comics, let's keep this in perspective, Nicholson's performance was and still is outstanding for the comic book film genre. On the other hand, a what goes for Ledger's take is that you don't see Ledger at all, you simply see the Joker, while Nicholson takes over the role: he was always Jack in everything he played since the early 80s.
Liam Neeson's Ra's Al Ghul is really Liam Neeson doing the Liam Neeson thing, which is great, I love his acting and his presence, but it's really nothing more than a well-known and respected actor lending his weight to the role, lending credibility to this comic book character and his motives.
Danny DeVito disappeared in his role, just like Ledger, and his work (helped by Stan Winston's makeup) is simply fantastic. Many people might dislike the film, but his performance just nails what his character and what this film is all about.
Michelle Pfeiffer - no comment, simply one of the best female characters in any comic book movie so far (and the best of the Batman films).
Christopher Walken is... Christopher Walken. Shreck is the true villain and catalyst of the movie, and no other actor could play this role with any kind of credibility. It's one of the most fun villains in the franchise, and his vampiric wardrobe and Rotwang-like haircut IMO classify as a fancy costume.
Jim Carrey did solid comedy work (for Carrey fans), it just doesn't do it for me as a villain. Tommy Lee Jones had nothing to work with apart from a bad looking costume and makeup. Arnold was paid 25 million. The gorgeous Uma Thurman is incredibly unsexy every time she opens her mouth to spout comments about Poison Ivy action figures or watery sex puns.
Cillian Murphy, Tom Wilkinson, Eric Roberts, all did solid work.
What about Vincent Schiavelli (one of the inmates in Cuckoo's Nest alongside DeVito and Nicholson, the train ghost from Ghost) and Rick Zumwalt (who arm-wrestled with Stallone in Over The Top before being blown apart by Batman)? RIP to them both
t: I love you, man. 
