Has to be Devil's Advocate. All the way through it is a brooding, actually quite horrible film dealing with insanity, suicide, lust and greed.

And then in no context whatsoever in the last fifteen minutes Al Pacino proceeds to overact his face off sassing it up as the Devil giving this overblown speech about how he's a "FAAAAAAAAAAAAAAAAAAAAAN O' MAAAAAAAAAAAAAAAAAAN". After all you've been put through, you end the film laughing uncontrolably at one of the most ridiculous scenes in cinematic history.