In 2017, the Big 10 had the #5, #6, #9, #18, and #20 ranked teams. In 2016, they had the #6, #7, #9, and #10 ranked teams. In 2015, they had the #4, #6, #9, #12, #21, and #23 ranked teams.
I'll admit 2017 was a better year for the SEC than 2016 and 2015. In '17 the SEC had #1, #2, #10, #18, and #19. However, 2016 and 2015 were very top heavy, especially 2015. Alabama was clearly the best SEC team in 2015; who was the second best? A 10-3 Ole Miss team who beat Alabama but also had losses to Memphis and Arkansas? Was it LSU or Florida, both of whom were convincingly beaten by Alabama? In 2016, who was the second best SEC team? LSU, who played Alabama close but had 3 other losses? 4-loss Florida who lost by 38 to Alabama? 4-loss Tennessee who lost by 39 to Alabama? 5-loss Auburn who lost by 18 to Alabama? You might think I'm cherry-picking teams from those years to prove my point but I'm not..those were all the other ranked teams in the conference. That's how you figure depth...who is the second or third best team in a conference?
The Big 10 not being the home of the champ doesn't preclude it from being the best conference overall in a given year. Was anybody arguing that the Pac 12 (then the Pac 10) was the best conference when USC was winning all their titles? It clearly was not...it was not a deep conference at all in those days, which USC was a beneficiary of.