La Sagrada Familia
This is my favorite church in the world, and it isn't even finished.  La Sagrada Familia was designed by Antonio Gaudi, and construction started in the late 1890's.  100 years later and this is all that was done (1993).  At that time, it was projected to be another 200 years to complete.  However, it appears they are using more modern construction methods, and it may actually be done by about 2025.  Significant progress has been made since I visited.