Tough choice. I went with 350 Boylston. The architecture of Fenway Center resembles the O'Neill Building after an earthquake, but it's at least rebuilding urban fabric over the Pike. Kingston is unlikely to get built at this point (at least as is), and Bromfield is ungainly but I'm not sure Downtown Crossing doesn't need more of these shots of life.
Of all the trashing of history these projects entail (or don't, in the case of Fenway Center), 350 Boylston's is clearly the most egregious. Replacing an intriguing 19th century building with a mediocre 21st century one is unjustifiable, particulalrly in Boston, and particularly on the rim of the Public Garden.
The 350 project more or less symbolizes everthing that's wrong with development in Boston, from the short sided knocking down of old buildings to the back room pay-to-play wheeling and dealings. All the other proposals, although bland (putting it nicely) will at least help their neighborhoods.