One that always goes down well with Germans is Drafi Deutscher's appalling "Marmor, Stein und Eisen bricht". (Do you like the grammar?) Another one, from the film "Große Freiheit Nummer Sieben" is, "Auf der Reperbahn nachts um halb eins", which has a wonderful chorus. That one is sung by old folks and young alike. It has also been covered by rock bands. It seems to be destined for folk song immortality. The other suggestions so far have all been very good. "Die Vogel Hochzeit" is also known and parodied unceasingly.