Recently I had a chance to visit the Art Institute of Chicago which is home to some of the favorite pieces mentioned above ("Nighthawks" and "A Sunday Afternoon on the Island of La Grande Jatte"). These pieces are both very impressive and I can see why anyone would choose to rank them very high on a list of favorite pieces. Both are large works and explore both style and atmosphere.
My personal favorites on my visit to this (truly impressive) museum were the Van Gogh and John Singer Sargent pieces hanging there, some of which I'd seen in books before and some I had never known about.
The simultaneous realism and stylized elements of both artists create an almost eerie other-worldly quality, making the paintings alive and active and present in a way that I've rarely seen.