The Apartment (1960)
Movie-wise, there has never been anything like it - laugh-wise, love-wise, or otherwise-wise!
Bud Baxter is a minor clerk in a huge New York insurance company, until he discovers a quick way to climb the corporate ladder. He lends out his apartment to the executives as a place to take their mistresses. Although he often has to deal with the aftermath of their visits, one night he's left with a major problem to solve.