As I mentioned in the cousin comment, I think the pyramids came in 2nd or tied, depending on how you count "standing." Their exposed stone is wearing much faster than the St. Louis Arch pillars sheathed in stainless steel, while they are also being buried by the desert. On the other hand, there's a lot of the Great Pyramid. At some point, however, the Pyramid is going to look like a big pile, while the pillars are still going to look like pillars. This article predates the fall of the World Trade Center, so my memory of it might not be 100%.
EDIT: I couldn't find the Sci-Am article, but it's interesting that the timeline of the Life After People television show, the middle of the St. Louis Arch is supposed to fall after 250 years, which matches the prediction of the earlier Sci-Am article. However, in the same timeline:
2000 Years: As it was after the ice age, Phoenix again becomes a vast savanna. Dolphins still remember their encounters with humans. Because it is basically held together by gravity, the medieval Cathedral of Notre Dame in Paris may still stand and be recognizable.
EDIT: I couldn't find the Sci-Am article, but it's interesting that the timeline of the Life After People television show, the middle of the St. Louis Arch is supposed to fall after 250 years, which matches the prediction of the earlier Sci-Am article. However, in the same timeline:
2000 Years: As it was after the ice age, Phoenix again becomes a vast savanna. Dolphins still remember their encounters with humans. Because it is basically held together by gravity, the medieval Cathedral of Notre Dame in Paris may still stand and be recognizable.