I think I can say that I have been to quite a few parks and Piedmont is the coolest by a landslide! It may have been because I went there in the fall when the leaves were changing colors and everything was gorgeous to look at, but I'm sure it is just as nice any other time of year too. Piedmont was huge and pretty diverse which made it a great place to walk. You can walk by the water, see the cityscape, swim in the pool, or bring your dog to the great dog park underneath the bridge. It may be strange to like a park this much, but if you ever make it to Atlanta, check it out and you'll understand!
Piedmont Park is fantastic! During the summer there are music conerts and movies on the lawn as well as art festivals with great pieces of art from sculptures to paintings to photographs and drwaing as well as other mediums. Very fun place to hang out or to walk or jog through during the morning or early evening.