Robert Downey Jr. is a great actor. When Quirky Downey Jr. was cast, I was quite skeptical. Then the movie came. In 2008, we were all blown away as he pulled off Tony Stark's swagger, cool, and intellect. Before Robert's great performance, the best Marvel performance was either Patrick Stewart as Prof. X or Tobey Maguire as Peter Parker, and maybe Chris Evans as the Human Torch. Robert has proven that he is a great actor, and a great comic book role actor. Ryan Reynolds has played Hannibal King, Deadpool, and (soon) the Green Lantern. Twenty bucks says that he'll get another role. So far Robert is superior.
Heath Ledger's Joker earned him an Oscar, and spoke volumes that a comic character could win. When Heath Ledger was cast as the Joker, I was so mad I smacked my littler brother. His performance made me want to slap myself. he embodied the role, and I would never have thought so before. Pre-"The Dark Knight," the best performances were Christian Bale as Batman, Jack Nicholson as Tim Burton's Joker, and Brandon Routh as Superman. I was a skeptic until these two performances. I now trust certain castings, e.g. Chris Evans as Cap and Mark Ruffalo as Bruce Banner.