Ok.. ABSOLUTELY funniest scene is Ace Ventura 2... when he's in the mechanical rhino. The rhino malfunctions, and he can't get out. Being in the middle of Africa, the inside of the rhino gets hot extremely hot, so he gets buck naked. Then he sees a hole at the back of the rhino, so he starts trying to dig his way out. So he's crawling his way out the ass of the rhino, naked and soak with sweat. Some tourists stop and point to their kids 'look kids, that rhino is having a baby.' Then they freak when they see a naked man coming out of a rhino's ass.
Another good scene is the American Pie scene when he gets it on with the pie. I knew that was going to happen because the commercial showed it, but damn, that was still the funniest thing in the world.