I have never managed to sit through Nine Month with Hugh Grant without laughing my head off / crying with laughter. I watched it when it first came out (I was about 14) and I have watched it several times and still laugh when I see it, Robin Williams is fantastic in it.
Also, I put The Longest Yard on the other night while DD wasn't very well and I didn't think I'd get much sleep. She slept the whole way through and I had to hold a pillow over my face as I was laughing so much. I watched it the next night with DH and still hurt myself by laughing at it!