Dirty Rotten Scoundrels
While certainly not one of his more famous movies, Steve Martin is absolutely hilarious in this movie, particularly in his "Ruprecht" character. Any Steve Martin fan who has not seen this one has to watch it, and you'll be watching it over and over again.
The 10 minute stretch where he plays Ruprecht - playing on his tire swing and lubing a surgical glove in the dungeon, wearing the eye patch over his eye while sticking a corked fork in his eye and urinating in his pants at the dinner table - that scene alone has me rewinding over and over, and has me in stitches every time.
Just may go queue it up right now.