This is perhaps the most famous building in the world. It takes up a full city block and there are always lines around the block to get it. Take the elevator to the observation deck and enjoy the views.
Unlike most architectural gems where you'll stand awe-struck looking at the exterior, Grand Central is one of those gems where the magic happens inside. As you walk into the main room, look up at the starry ceiling, and make sure to go downstairs to check out the echo chamber.
The first tower to be built in NYC after 9/11, this building is also the first "green" high rise office building in the city. When you walk by, make sure to go inside and check out the enormous waterfall in the lobby atrium.