I would say Chicago because they seemed to have the toughest start to the season with games against Dallas and Green Bay. While Dallas started off slow, the Packers looked great through 2 games.
I think the hype around the Chiefs might be a little premature. They beat a wildly inconsistent Chargers team, and two bad teams in the Browns and 49ers. They're on the road against the Colts and Texans for games 4 and 5. They'll be 3-2 after that.
Also, the Steelers haven't had that tough of a start, and with all the talk about Big Ben being out, that team is still all about defense. So I'm not that surprised by their 3-0 start.