I'm rewatching One Flew Over The Cuckoo's Nest. One of my favourite films of all time. In my opinion, while Jack Nicholson evidently steals the show in this one (as in every other film he's in), this film has one of the strongest cast ensembles, from the nurses to each one of the mental patients.
Playing a mentally ill person must be one of the hardest challenges for an actor. I remember Leonardo DiCaprio killing it in Gilbert Grape (I hate him now, but let's not forget that he used to be good), Vivien Leigh in Streetcar, Dustin Hoffman in Rain Man, Rani Mukherjee in Black, Mr. Os in Adik, and of course, the mighty Sean Penn in I Am Sam.
I refuse to put in films like American Psycho and As Good As It Gets in the list, as I don't really find the main characters crazy. In American Psycho, Christian Bale is a very well-groomed and well-mannered psychopath, and psychopathy is not really a cuckoo-kind of mental illness, IMHO. And Jack Nicholson in As Good As It Gets is just obsessive compulsive, and that's not really a mental illness as much as a personality tic. Although, I'm no shrink so everything I say is from a layman's POV.
One Flew Over The Cuckoo's Nest is one of the films I would want to keep in my future movie room (of course I will have a movie room in my future 3-acre mansion) and watch every other Saturday night with my kids so they will grow up better than other kids whose cinematically-challenged parents make them watch cartoons all day long.