The Weismann Museum in Minneapolis, MN USA - by Frank Ghery.
(I'm generally not a fan of Ghery's designs - to me, once the initial shock and awe wears off, I like them less with each sighting..... (I know that's heresy for many))
The Tower of History in Sault Ste. Marie, Michigan. It just doesn't fit at all with the landscape of the other buildings in town. The only upside is that it gives a great view of the Soo Lochs.
I'm sure someone will suggest Tour Montparnasse in Paris
Though frankly, I don't understand why everyone hates it. Maybe it doesn't live up to Parisian standards of architecture, but there are thousands of buildings just like it all over the world, particularly in North America, including in the nearby La Defense.