Right, here are mine so far.
1) The Joker- Johnny Depp
I think that The Joker's monologue at the end of TDK makes it essential that the character recurs throughout the films. Now, how best to do this whilst complimenting and flattering Heath Ledger's stunning performance, and the man himself? Byt giving the part to an widely lauded actor with nothing to prove.
2) The Riddler- Michael Sheen
Sheen is a fantastic actor, able to assess and reproduce all of the nuances in voice and manner of several famous people. He is also able to bring depth and inventiveness to fresh roles. As a physical type, his wirey, rattish, boyishness suits the character well.
3) Catwoman- Emily Blunt
Blunt is just great, and everything about her doesn't so much screem "Catwoman" as purrs "Selina" down my ear.
4) The Penguin- David Suchet
Seen here as the fraudster Robert Maxwell, Suchet is a screen-stealing character actor who is highly adept at projecting a calculating intelligence and a malice much larger than his limited height.
Black Mask- Mark Strong
I just love this guy. His performance as a Reggie Kray-like Harry Starks in series "The Long Firm" was magnetic in its malevolence, as was his character in "Stardust". One of a very few actors I would trust to bring a wooden black mask to life.