I have several favorites:
1)Tommy Boy - just about every scene, but a memorable one: When Chris Farley breaks the door on David Spade's car by driving forward at the gas station with the door open. He closes it and pretends nothing happened. When Spade comes along and opens the door, Farley shouts "What did you do?!?"
2)Planes, Trains, & Automobiles - "You're going the wrong way!" Nuff said.
3)Honeymoon in Vegas - Sarah Jessica Parker-"LIKE a straight flush is NOT a straight flush" Nicholas Cage-"Hey! I know that now!"