The (Once) Biggest Cuckoo Clock in the World
Even tough it says 'Die größte Kuckucksuhr der Welt' in the clock, it only held this name for a while after it was built in 1946 (source: wiesbaden.de). But it still is the biggest 'of this type', whatever that means...
I would say that even greater sight is the souvenir store by which this clock is located. There are thousands of small souvenir and gift items by the windows and even more inside. Every time my mother walks pass the store, she says quite loudly 'ooh!'.
And you must not worry: if you are not coming to Wiesbaden anytime soon, you can always shop online: http://www.gifts-from-germany.com/american/index.html