For a non-chain - Holeman and Finch here in ATL. But you can only buy it after 10PM
http://www.holeman-finch.com/
And if you could make it travel well (which you can't) then you'd combine it with fries from Shaun's which are cooked in duck fat.
http://www.shaunsrestaurant.com/
Besides In and Out, Whataburger is my 2nd choice for best chain outside of ATL:
http://www.whataburger.com/
And 5 Guys (they have vinegar for your fries too....) inside ATL:
http://www.fiveguys.com/home.aspx
But I personally like my hamburgers the best
