We probably should preface this thread that what one considers ugly architecture/design, like everything, is entirely subjective.
For me, I've started to like and appreciate Boston City Hall over the past couple years as I've grown a taste for mid-century modern, and then onto even a bit of brutalism. No one can argue that the public space is any good around Boston City Hall, or most brutalist structures for that matter, but the design has grown on me as an art form. Same goes for the lattice covered Ruth's Chris building too.
I'd say buildings that try to be something the architect has no familiarity with, and it shows, are some of the ugliest. There is a lot of nouveau historical that fits into this category. Or designs that are 20-30 years outdated, but being designed as new construction today. Or something that blends architectural styles into an ugly, non-conforming soup. Case and point:
Source - From LUAR PDF