What are your standards to determine toughest? And are you gauging teams/programs or conferences?
There's no question Oklahoma is the top program right now. Florida is right there with them. There is a big gap between 2 and everyone else. Louisiana is certainly one of the top 20 programs, possibly top 15.
As far as conferences are concerned, I understand that people use "what have you done for me lately" as a measure of success, but that measure is still high enough to calculate that there isn't much difference between the two conferences.