First, let's look at just the average ratings for the conferences.
The SEC has a slim lead over the Big-12 with the Pac-12, Big-East, and Big-Ten in a group behind. If we look at this same average for the SEC East and West we get:
A pretty big gap, but even the SEC East would be slightly above the Pac-12/Big-Ten/Big-East.
Another way of looking at conference strengths is to look at performance in non-conference games. But one can't just look at records as some schools/conferences play a lot of FCS/I-AA teams or even weak FBS/I-A teams. A better way is to group the games played by a conference and run them through the computer's algorithm. Doing this yields:
Interestingly, the Big-12 comes out on top here narrowly over the SEC. They have similar records and schedule strengths so it is very close. They haven't gotten to these records playing the toughest schedules though as only the Big-Ten and ACC have weaker out of conference schedules.