My favorite is the one right on top of the metro seen in these pictures, the New Post Office (now the old post office!) also known as the Ariel Rios Federal Building. This was designed by one of my favorite firms, not surprisingly at all, Delano & Aldrich and was finished in 1935. This was built as the core feature of all the buildings and was modeled on the Place Vendome in Paris.
According to this informative website HERE, the central section of the building is comprised of two huge back to back, semi-circular units with side wings. The 1/2 circle formed by the building's curve was to be mirrored by a similarly curved façade built across the Street on the site of the Old Post Office Building, seen below.
The Old Post Office, which has magnificent views of the city from the tower and is always full of tourists, was fortunately saved from the wrecking ball in 1970 and is now private offices. However, the rest of Delano and Aldrich's plan for a main circular court in the federal triangle was never completed because of this. Half of the court exists anyway for us to enjoy! I hope you enjoyed this tour and my photos!