In my opinion the ESB and the Chrysler Building aren't "that" iconic only because most people get them confused, unlike the rest of us who know something about buildings.
So in my opinion "Iconic Structures" would be things like (what my girlfriend would recognize)-
The Eiffel Tower
The Great Pyramids of Giza
The Statue of Liberty
The St. Louis Arch
The Liberty Bell
The Original World Trade Center (Possibly the new World Trade Center as well)
Big Ben in London
The Golden Gate Bridge
Hollywood Sign
Times Square (More then one building of course)
Taj Mahal
The Colosseum
Stonehenge
Great Sphinx
The Vatican
The White House
The U.S. Capitol Building
The Great Wall of China
Mount Rushmore
The Washington Monument
Yada Yada Yada...etc etc
Buildings that come in at a close second would be (meaning people have a general idea of what they are, or people who know something about buildings or architecture would recognize)
The ESB
Chrysler Building
Sears Tower (Willies Tower)
CN Tower
Bank of China Tower
Transamerica Building
The Burj Khalifa
The Metropolitan
The Philadelphia Museum of Art (Rocky Stairs)
Hagia Sofia
The Louvre
Sydney Opera House
The Burj Al Arab
Seattle Space Needle
Dare I also include One and Two Liberty Place?
Bank of America Building?
New York Times Building?
Philadelphia City Hall?
Independence Hall?
Grand Central Station?
Penn Station?
Madison Square Garden?
I think I compiled a decent list lol