Antonio Salieri: F. Murray Abraham absolutely NAILED it in his performance, even if certain creative liberties were taken as far as the actual composer goes.
Lionel Logue: Again, Geoffrey Rush totally killed it in this role even though it might not have been 100% faithful to the actual historical figure.
"Red" Redding: Pretty much anything Morgan Freeman touches turns into gold, but this might as well be platinum. Even though he plays a convict, he does a brilliant job of making the audience actually empathize with someone with such a troubled history. His performance says it all - that the irredeemable can truly be redeemed.
Dorothy Gale: The Wizard of Oz - even 80+ years later - still remains the gold standard for the fantasy film genre. "Somewhere Over the Rainbow" still gives me chills to this day. Hard to believe Judy Garland was only 15-16 years old when this was filmed in the late 1930s.
The Joker: Batman's archnemesis remains arguably the most famous comic book supervillain. That's due in no small part to the many different portrayals he's had in television and film. There was the campy 1960s version played by Cesar Romero, the Jack Nicholson version in the late 1980s Tim Burton classic, and the criminally-underrated (no pun intended) voice acting talents of Mark Hamill gave the character new life in the highly successful 1990s animated series. But it's the Heath Ledger version that wowed audiences. Gone was the bumbling prankster, gone was the murderer of Bruce Wayne's parents. This Joker was unlike anything we've seen before. He's evil, yes, just like the other renditions - but in The Dark Knight, Ledger's Joker explores new territory. He's chaotic, he's sadistic, and has no regard for human life - including his own. He's clearly insane but he's also got a certain cunning and genius that previous versions lacked. His only goal was to create as much chaos as possible - he wanted people to live in abject fear. Out of respect to Ledger, I'm glad they kept the character alive even though he clearly doesn't deserve to live.