Having only been to two, I can only say that Six Flags over Georgia is considerably nicer than Elitch Gardens (which was owned by Six Flags when I went there years ago).
I'm a little intrigued as to why most of you who've been to SFoG are ranking it comparatively low. I thought it was pretty scenic and clean, with a well-rounded selection of coasters. It certainly exceeded my expectations for a Six Flags park.