The entirety of the scene where John Belushi in Animal House traverses the cafeteria line, piling food on his tray, pocketing a bunch of bananas, shoving an entire burger into his mouth, and then furtively looking around before slurping down some jello and putting the plate back on the dispensing shelf. I laughed so hard at that.
Porky's - gee, where do I start? The lineup scene out in the 'glades when the big dude burst through the window waving a machete and yelling. When the guys are peeping on the gals in the locker room shower and the word "lardass" comes out of PeeWee's mouth. When the assistant basketball coach is trying so hard to stifle his laughter when the other coach is boning Kim Cattrall in the boy's locker room. And probably funniest of all, when Ms. Ballbraker is talking to the school principal with all the coach's present, she is describing the incriminating mole on one guy's pecker, and the principal wants her to refer to it as a tallywhacker. Finally the coach who did Kim Cattrall suggests bringing in a police sketch artist to draw up a poster saying "have you seen this *****?" And the principal, who noticeably straining to stifle his laughter, finally bursts out in raucous laughter. I think by then I was literally rolling on the floor LMAO.