Technically, NO league has an auto-bid. It's the top five ranked conference champions that get a bid.
Obviously, the B1G, SEC, B12 are 100% of the time always going to get there. The ACC usually, too, but that's more tenuous than they'd like. Then that leaves one for the top ranked G5 conference champ, which I think happens much more often than the PAC would like, will be the AAC.