In St. Louis...
I would have picked the Kiel Opera House, but it's beginning renovation.
Municipal Courts Building has a renovation plan, though nothing has started.
A few others are still empty - Arcade Building, Chemical Building.
A lot of North St. Louis is in jeopardy, and has been destroyed in the hands of Paul McKee.
Some St. Louis School buildings are under threat.
And there's Dillards and the St. Louis Centre Mall.
There have been a few closed churches that have seen some damage and are in danger.
If I picked a part of the city in most danger, I would choose North St. Louis.
As far as a specific building, I can think of a few that are probably in danger, but there's not currently one building that jumps out at me.
Of this:
http://builtstlouis.net/crumble00.html (http://builtstlouis.net/crumble00.html1 - broken link). City Hospital is now the Georgian Condominiums.
2. The Arcade Building is still empty.
3. The Syndicate-Trust has been renovated.
Syndicate - Located in Downtown St. Louis, MO 63101
4. The Kiel Opera House is undergoing renovation
5. I don't know much about Shriner's Hospital
6. As far as I know, Carr School is still crumbling
7. I believe Cass Avenue Bank is still in trouble.
8. Paul McKee has plans for the Clemens house, but McKee has the track record of letting homes he owns crumble.
9. The Avalon theater is facing demolition, and no one seems to know who actually owns it.
11. The San Luis is safe for now.
12. I believe MacArthur Bridge is still in danger.
But no one building sticks out to me.
In my little town, there's a 2-story red brick building which hasn't been used in decades. I wouldn't be surprised to see it torn down.