First, let's start with the chosen subject: In One Flew Over the Cuckoo's Nest, Randall Patrick McMurphy, a common criminal, has his sentence commuted from jail to a psyche ward. Unhinged but clearly sane, McMurphy makes his mark on the other patients and the staff at the hospital, especially his nemesis, Nurse Ratched. This movie is fantastic. The characters and the actors who portray them are engaging, entertaining, and realistic. At it's core is an incredible performance by Jack Nicholson in what I think is the third greatest performance of his very noteworthy career.
His greatest performance, in my own humble opinion, is in As Good as It Gets. Here his craft is honed and precise, his character filled with quirks but 100% believable. The movie seems like it was written just for him, like the entire effort was put in place so Jack could bring Melvin Udall to life. Like many of the movies I've mentioned, this is a mix...It's a hilarious comedy, but it has a hell of a lot of heart and is a poignant drama at the same time. If you have not seen As Good as It Gets, you should go see it. Now.
Nicholson's second greatest performance is as Jack Torrence in The Shining. It evokes of one of his lines from As Good as It Gets: "Go peddle your crazy somewhere else. We're all stocked up here." The Shining is chock full o' memorable moments. Like when he's stalking his wife up the stairs, demanding the baseball bat, or when he pokes his face through the door and says, "Here's Johnny!" Few people can turn on the crazy like Jack Nicholson.
And I'll close out with an honorable mention: He made The Joker an icon before Heath Ledger did. He totally made Time Burton's Batman, again amping up the insanity and giving an inspired performance that stole every scene. I won't compare his performance to Ledger's because they are too different, it's like trying to pick my favorite ice cream (Ben & Jerry's Chubby Hubby) over my favorite potato chip (Ruffles)...they are both totally delicious, one's sweet and salty the other is pure salt, so it depends on what I'm in the mood for.