Interesting question:
Ryan Reynolds: Liked him as Deadpool, was meh on him as Green Lantern. Instead of Hal Jordan, I wish he had been cast as Wally West in a Flash movie.
Parker Posey: Didn't like her in either Blade Trinity or Superman Returns. Though she might've made a better Lois Lane than Kate Bosworth did.
James Marsden: I thought he was okay as Cyclops, but after his good turn in Superman Returns, I would've rather seen him as Superman over Brandon Routh.
Kevin Spacey: Though his Lex was inspired casting, until I saw the film. That being said would've liked to see would he could've done with Justin Hammer from Iron Man 2. Sam Rockwell exuded no menace at all in that movie.
Alec Baldwin: I thought his Shadow was okay,but I would've liked to see what he could've done as either Batman or Two-Face in one of the later 90's Batman films.
Tommy Lee Jones: Horrible as Two-Face, but due to the comics, I would like to see him as Norman Osborn for the new round of films. (I really did like Willem Dafoe in the Maguire films).
Ciaran Hinds: Completely wasted in Ghost Rider 2. Would've liked to see him as Justin Hammer too.
Idris Elba: Wasted in Ghost Rider 2, would like to see him as Black Panther, Bronze Tiger, or somebody headlining a superhero film.