The building I'd most like to see obliterated (or at the very least thoroughly overhauled) in L.A. is the
Westin Bonaventure. I do like the futuristic five-tower configuration somewhat, but that massve lobby/atrium/mall/podium/parking garage/fallout bunker of a base is just a heinous sin of (anti)urban design. The interior seemed confusing, humid and dimly lit, as well, the last time I visited, but I may have been in a grumpy mood...
At least I know where to be if LA ever gets nuked...
.........
Also, though I'm not from New York, nor have I yet been able to visit, I love the city, with about 6 exceptions.
I just think the South Street Waterfront would've looked SO much better before being buried by those immense boxes. It's not that they're ugly, per se, but that they deformed a really awesome mountain-like skyline, and tore up the Downtown urban fabric. See what it was like in 1959:
(sorry, no idea who's got the copyright on this one... but I love it)
Oh, well. I'm still dying to see NY, though.