All of the Carl Reiner-Steve Martin collaborations are extremely funny in my opinion. The Jerk, The Lonely Guy (Charles Grodin is fantastic in that one), Dead Men Don’t Wear Plaid, and The Man With Two Brains ( you’ll never guess the identity of The Elevator Killer).
However, the movie that made me laugh the most was The In-Laws, with Alan Arkin and Peter Falk.