My favorite ones are/were:
WVU: Always a great game. Always meant something big to both teams.
UL: We'd trade wins back and forth with these guys.
Buttgers: No matter how bad they were year after year, Schiano always had our number. These guys broke our hearts so many times, that the rage/hatred still lives in me.
UC: Another conference mate dating back to the old days. Always a great game, and the annual "chili cookoff" tailgate will be missed.
Central Orlando: I know many of y'all will say no to this one, but yes. If you are passionate about not listing them as a rival, then you are lying to yourself...You are not indifferent. You HATE them. They HATE us, and we HATE them, and no matter what conference they go to or what conference we are in, we will continue to HATE them.
EDIT: right after I posted this, I realized that every one of these teams would most likely mop the floor with us right now. . Sad.