The Gators. I'm a younger UT fan (born in 86), but I've always had a deep-seated hatred for them.
It was really cemented when I went to the UT-UF game this year and sat in the UF section.
I hate Alabama too, but I respect the fact that their fanbase is at least passionate, travels well, and has tradition backing them. The Gators fanbase is composed almost entirely of bandwagon fans who have no allegiance to the team and were nowhere to be found during Meyer's last year and Muschamp's first.