Other than Georgia, over all? Florida State. Before the scheduling change, I had a severe and profound hatred for that bunch. I imagine it'll be rekindled this year.
Couple of other comments:
School we play every year? Clemson. For 359 days they're sort of our brotherly/friendly rival and that's fun. But that one week, I want to stomp them to death. I used to want a close, fun game but after the Purple Shirt Massacre in 2003, I just want to beat the hell out of them.
Auburn is my "Other" option. The three year stretch from 2003 to 2005 was great fun, especially because I happened to be around alot of Tiger fans during that period. The atmosphere with them is different because the game itself feels like playing Clemson, but the fans can be closer to Georgia fans.