Currywurst. Chopped fried or grilled sausage with ketchup and curry spice sprinkled on top. Often served with fries.
You can get it almost everywhere in Germany, especially at street festivals. Simple, absolutely unhealthy and delicious.
Edit: I would also have said the Döner Kebab. Veil or chicken grilled on a vertical spitroast, sliced into thin strands of meat, loaded into a slightly toasted flatbread along with lettuce, tomatoes, cucumber, onions and depending on the region and restaurant white or red cabbage in vinegar and oil, together with a yogurt sauce.
But you could argue that Döner is Turkish because it was invented by a Turkish immigrant and is usually prepared by Turkish descendants (or those who look Turkish). But then again I heard that restaurants in Turkey started offering German Döner because that's what tourists expected to get.