John Houseman as Professor Kingsfield in The Paper Chase.
Ted Knight, Rodney Dangerfield and Bill Murray in CaddyShack
I’m willing to give Christensen a certain amount of slack on this one, since he was having to act under the direction of George Lucas, who is pretty well-known to provide his (flesh and blood) actors with very little in the way of useful direction.
I’m not sure that any other actor of Christensen’s age and experience level could have done much better. Before being cast as Anakin, Christensen had gotten quite a bit of acclaim (including a Golden Globe nomination) for My Life as a House, and he later got several award nominations for Broken Glass. He’s not a bad actor, but the Star Wars movies were not going to be where he got the direction to shine – for that matter, the Prequel Trilogy was filled with actors who were well-regarded for their other work, but most of them turned in fairly wooden performances in these movies.
Perhaps too recent to judge clearly, but Kevin Spacey in the US version of House of Cards… I simply can’t imagine another actor bringing half so much to that role. Lester Burnham with power.
As for miscast, I think a singing Sly Stallone speaks for itself. His mother should have shot him.
Frankly, I don’t expect Oscar worthy acting in a comic book movie nor am I much of a hard core geek. (How dare you! :D) But if fidelity to the original looks aren’t that important then why the need to transform her into a blond Nordic. Why not just say Susan Storm is now a brown-eyed brunette?
Raul Julia and Anjelica Huston as Gomez and Morticia Addams. Special mention for Christina Ricci as Wednesday Addams. Two movies that worked almost entirely because of casting alone.
Worst - well, I risk the Wrath of Skald here, but I thought Miranda Otto was a terrible choice for Eowyn. Too little, too pretty, too girly, and couldn’t swing a sword to save her life. Needed to cast Dernhelm, not Eowyn. She’s not a bad actress, and I’ve enjoyed her in other movies, but a bad, bad casting choice for that particular character.
A question more movies need to ask themselves when working from a written source. It isn’t as if Alba wasn’t beautiful as-is, nor was her character’s super-power being blonde. Just let her be a brown-eyed brunette.
And I would have no problem with that.
And, a million OCD fanboys suffer from exploding craniums. 
And Matchstick Men! I love Nic Cage’s crazy ass i don’t even care
Obvious one but Natalie Portman in V for Vendetta was a terrible decision. And I don’t know if anyone has seen Girls but Chris O’Dowd’s American accent is DISTRACTINGLY bad.
I think that was a script problem, not an acting problem. As Mika said when the movie came out, Superman is not a deadbeat dad. And, even powerless, he’d have put up a better fight against those three thugs. Anyway, the worst casting in that movie was the yougn woman who played Lois. Kate Bosworth, I think. She simply looked far too young to be an award-winning reporter AND mother. She looked about fourteen, tell you the truth.
I don’t have time to chastise you for your impudence today, Mrs. Cake. Please call Operations to schedule a vicious wet-willying.
Jimmy Stewart in “It’s a Wonderful Life.”. You believe he would do all the stuff George did for his family and friends. He’s also good at the stress-induced freak out. And of course you believe everyone loves him.
The guy that plays Joffrey in Game Of Thrones deserves special mention in the “Best” category, IMHO. For one thing he plays 14 pretty convincingly for a guy in his twenties. He also just looks like the kind of guy that would be an utter dick.
Timothy Olyphant in Justified. He can narrow his eyes and talk in a menacing whisper, but I don’t buy him as a child of Kentucky, let alone a tough guy. He’s too slight, too pretty, and his skin is too perfect. He looks like a Hollywood actor pretending to be a tough guy.
Best:
Robbie Coltrane in Cracker was bloody amazing. No one else could play that role.
George Clooney in From Dusk 'til Dawn was just fun to watch. Probably the only film Tarantino has been in which I actually though he suited his character.
Joe Pesci in Goodfellas
Val Kilmer in Tombstone & Kiss Kiss Bang Bang
Worst:
Eddie Murphy in pretty much anything since 1990 (Donkey in Shrek being the exception).
Yep. He did okay in the first two seasons of Deadwood, and then became part of season 3’s many problems by turning into a sort of self-parody, capable of nothing but standing around looking pissed off. He carried that flawed portrayal into Justified, IMHO, as much as I otherwise like the latter show.
Tommy Lee Jones in Natural Born Killers always stands out to me for some reason, he played his role so well.
Andreas Katsulas as G’Kar on Babylon 5. He had the gravitas to shine from behind the makeup, and nobody else could have pulled off the sheer joy of the part. OTOH, Michael O’Hare was horribly cast as Sinclair. The bulkheads had more character.
In **The Wire **I thought most of the thugs were well cast, particularly Omar, Slim Charles and Marlo Stanfield. They had the right look and the acting skill to create believable characters.
If I’ve read the novel, or studied the actual person from history, I may have disagreement with the actor who’s cast for box office appeal
Khartoum from 1966 suffers so obviously from Olivier in blackface that Charleton Heston’s miscasting as Gordon isn’t as noticeable. Omar Sharif (who was busy being miscast as Zhivago) and John Mills would have been so much better.
I never really cared for Nancy Marchand as Livia in the Sopranos. She could have been playing a mean old WASP in Minneapolis or a mafia matron, for all I could tell. Laila, (who is from Minneapolis) who played her in the flashbacks, nailed the part.
Of course, Sian Phillips as the original Livia in I Claudius is the gold standard for best cast actress.
I’ll see your Hayden Christensen (which I agree with) and raise you one Andie MacDowell in Four Weddings and a Funeral. Way to shit all over an otherwise fun movie.