Looking at average ratings top to bottom, he SEC and Big-12 are far and away the leading conferences but it is closer than you might think. The Pac-12 is a comfortable #3 with the B1G, ACC, and Big-East fairly close a few points back.
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.