Basement usually because most people are afraid to go down there!
Well, if the house only has 2 floors, you won't have anybody walking over your head on the 2nd floor, so I'd have to say that one.
WTF kind of question is that? Do you have concrete floors? Are your return air ducts on the 2nd floor really just holes in the floor to the first floor ceiling? Are the people on the 2nd floor always playing music? Does it constantly rain on the metal roof you may have? Are the floor joists on the first floor creaky? Is your furnace in the basement old and noisy? Are the people on the second floor extremely obese causing them top stomp everywhere? Is the house next to a very busy street? Do you have loud neighbors? Without any more information, I would say that the quietest place in the house is in the bathoom, in the bathtub, underwater,as you drown yourself.
2nd floor if the floors creak or if you have an open concept layout for general living area; basement if you live in the city or tend to have a lot of wind/weather. First floor is only quiet if everyone tends to hang out in their bedrooms or in the basement (like if there's a media room in there).
