Any kind of a joke indicating that we are Not the guardians of Liberty and Peace tends to get our dander up. Or when the French kid us, saying we would still be a British Colony if not for Lafayette. We also hate all forms of humor which imply that our automobile manufacturing is sub-standard. Also, snide jabs at the Kennedys are strictly taboo. And please stay away from the "fast food" humor.
Actually, Americans are quite sure that we are at least as silly as any other country on the planet, and we would welcome any additional jibes that we haven't already come up with.*
* Except unwarranted sarcasm from the Canadians, who are just jealous.