The only answer is a young WR, likely Trader and/or Carr.
I know Beck is good, I know the line is good, I know the backs are good, I'm very confident Lofton will be, at worst, pretty good.
If Trader or Carr truly breaks out, this will be a Top 15-20 offense again and we're going to be a very good football team.
I can see if someone wants to give one of the Safeties an honorable mention, but if you're answering this question in terms of what makes the team the best it can be, it's Trader or Carr (or really any young WR) blossoming into someone headed towards being a high NFL pick.