Best shots I ve seen of the skyline:Atlantic Station Parking Deck: Incredible viewOverpass above Freedom ParkwayThe Overlook in SmyrnaAny tall building in buckhead...looking south
I agree with the park, great location for photos of the city skyline. You might also want to check some of the parking garages around town. A view from the top of one of the garages may be ideal.The deck at Lindbergh City Center has a great view of the Midtown skyline.
Piedmont Park is awesome for that...many people get their engagement pictures taken there with the city in the background.
Try on of the high rise building in downtown or from Kennesaw Mountain or Stone Mountain.