Bad Santa (1994)
He's not fat, he's not round, and he definitely isn't jolly. Billy Bob Thornton's booze-drinking, womanising Santa Claus is hilarious, in a dark comedy sort of way.
A strange friend in the form of a socially inept boy called Thurman Merman makes the off-the-rails Santa see the error of his ways after years of drug abuse and robberies. Heartwarming and filthy at the same time and therefore probably the ultimate alternative Christmas movie.