It's tough to argue right now that the Denver Broncos are not the best team in the NFL. Yes, they aren't in first place in their division, but that's simply an artifact of this year's schedule; San Diego is ahead of Denver because San Diego hasn't had its bye week yet. But the Broncos are 4-1 against a much tougher schedule than San Diego has faced, and their only loss came on the road in overtime against the defending champion Seahawks.
More importantly, the 2014 Denver Broncos are a great all-around team, more well-rounded than last year's AFC champions. Peyton Manning and the Broncos offense have not started off with the record-breaking pace of 2013, but they still stand as the best offense in the league according to the Football Outsiders DVOA ratings. But perhaps more importantly, the Denver defense has been almost as strong. The Broncos currently rank second in the Football Outsiders defensive DVOA ratings, trailing only Detroit. (DVOA, or defense-adjusted value over average, is explained further here.)