Kevin Costner as Robin Hood.
George Clooney and Val Kilmer as Batman.
good nominations
Just about any native american role in the 50's such as Burt Lancaster, Rock Hudson, and Roman Gabriel
agreed. they weren't believable when I was a kid, and now I can't watch any of them.
Tony Curtis as Myles Falworth in "The Black Shield of Falworth." Runner up: the very same Bernie Schwartz as Antoninus in "Spartacus."
Can't disagree with either nomination.
As flat_cap indicated, there are sooooo many from which to choose.
Some other brutal castings, in no particular order:
1. Neil Diamond in the remake of The Jazz Singer
2. Mickey Rooney as Mr. Yunioshi in Breakfast At Tiffany's
3. Cindy Crawford as Kate McQuean in Fair Game
4. Demi Moore as Hester Prynne in The Scarlet Letter
5. Keanu Reeves as Buddha in Little Buddha
6. Melanie Griffith in A Stranger Among Us
7. Sofia Coppola as Mary Corleone in Godfather, Part III
8. Denise Richards as Christmas Jones in The World Is Not Enough (pretty to look at, but did anyone believe she was a nuclear physicist?)
9. Colin Farrell as Alexander the Great in Alexander
10. George Lazenby as James Bond
and I'm not even scratching the surface.