Smoothest riding small car Ive ever been in is a Renault R5 (LeCar in the US).
Seeing as how French Cars are rare in the US I would try some Korean Cars as they tend to be more softly suspended than Japanese Cars.
I cant really think of an American Subcompact I would consider as smooth and quiet. The 2.2L OHV GM cars are pretty good on fuel. My little sisters 2000 Sunfire routinely got 30 mpg city and 36 mpg highway at 75 mph. Thats with no regard for FE driving and an automatic (although an uncommon 2.2 OHV / 4-Spd Auto combo).
PS A lot of "smooth" on small "square edged" bumps has more to do with tires than suspension. Soft sidewalls and low air pressure are what absorbs these kind of bumps.