Inspired by a recent comment by MD I think this may be a decent post. Which roles throughout the history of movies had the perfect actors playing them and conversly what roles had the worst casting.

Of course you have the obvious ones where the actor pretty much made the role like the most obvious Sean Connery/James Bond. But it gets a little harder with already established actors playing high profile characters. Here's some good and bad to start ya off.

Good:
Kevin Spacey - Lester Burnham in (American Beauty)
Christopher Loyd - Doc Emmett Brown (Back to the Future)
Bill Murray - Dr. Peter Veikman (sp) (Ghost Busters)
Tobey Mcguire - Peter Parker/Spiderman (Spiderman)
Johnny Depp - Hunter S. Thompson (Fear and Loathing)

Bad:
Sean Connery - Alan Quartermain (League of Extrodinary Gentlemen) I guess you could say everyone in that one
Mark Wahlberg (sp) - Captain Leo Davidson (Planet of the Apes)
Tom Cruise - Nathan Algren (The Last Samurai)

The bad seem to be a little harder then the good to me at least.

Good

Brad Pitt - Tyler Durden (Fight Club)
Uma Thurman - The Bride/Beatrix Kiddo (Kill Bill)
Bruce Campbell - Ash (Evil Dead)
Bill Paxton - Any role in a James Cameron film (except Titanic)

Bad

Jack Palance{RIP} - Yves Perret (Tango and Cash)
Julia Roberts - Tess (Ocean's 11, 12, 13?)
The Entire Cast - (The Core)

Good casting:

• Robert de Niro - Jake Lamotta in Raging Bull.
• George C. Scott - Patton
• Harvey Keitel - as the lieutenant in Bad Lieutenant. 
• Alistair Sim - Scrooge
• Tom Berenger - Sgt. Barnes in Platoon
• Clint Eastwood - William Munny in Unforgiven.

Bad casting:

• Jim Carrey - Peter Appleton in The Majestic
• Heather Graham - Felicity Shagwell in The Spy Who Shagged Me.
• Kevin Costner - Robin Hood
• Keanu Reeves - as the Buddha in Little Buddha.

Yeah good call on Costner. I guess Kevin Costner as robin hood has got to be one of, if not THE greatest miscasts of all time in my opinion.