For me, the number one would probably be Twinkies. They're frequently mentioned on a lot of American based TV shows / Movies I've seen. They've been 'advertised' to me so much through said mediums that if I'm ever lucky enough to go to America, trying one is certainly on my 'to-do' list.
To a lesser extent, despite them being German foods (correct me if I'm wrong), foods like hamburgers / hot dogs (probably from hearing about the copious hot dog stands in NY etc.) spring to mind.
Coke, too.
Hmm...It's all unhealthy; perhaps I'm buying into the stereotypes a little