My vote goes to that combined Le Corbusier-Maekawa monstrosity in Ueno Park, the National Museum of Western Art. These are two of the finest architects of the 20th century, but Corbu screwed the pooch in 1959 and Maekawa didn't do the place any favors with that horrid abomination from the late 70's. As a comparison, check out Maekawa's 1940s house, lovingly preserved at the Tatemono En, which has been discussed here in FT before.