Best is such a personal thing.. depends what people value more. Many people worship the QB position because the game is so dependent on them, but there are 10 other guys on the field as well making them who they are. Fantasy Football certainly skews this question for people as well.
The best in the game is tough. There is no definitive best since each position calls for such unique skill sets.
Peyton Manning, Antonio Gates, Wes Welker, Chris Johnson, Jared Allen, Jon Beason or Darrelle Revis. Can't name an offensive lineman since they are faceless, nameless, bad-ass stockades for their respective teams. I can't pick - please don't force me to!