I have a hard time accepting the notion that Rivers is the best QB in the division after the way he played last season. Admittedly, Manning was out of the game altogether at the same time, so it's hard to really judge, but Rivers has never been better than Manning. I don't see any reason to think that has changed.
That said, I think this division is looking very unpredictable this season to be honest (other than the Raiders sucking--that's almost certain). The Broncos, Chargers, and Chiefs all have pieces in place, but none of them look even remotely like complete teams. So in the end it's probably going to boil down to who has the fewest injuries and the most lucky bounces.