Well I'm going to assume the poster meant, who would you add right now, to win right now, not looking into the future several year when Manning is gone.
Right Now - who would I add that I think would take Denver to another level?
Adrian Peterson - without a doubt. He would make the run game special and most teams would have to keep a safety in the box and would not be able to play nickle and dime often. That would be open season for Manning and his WR's.
Next I would have to say Patrick Willis. A stud MLB would make this defense even better. He can cover TE's, stop the run, rush the passer, he does it all. And every down backer that doesn't make many mistakes.
After those 2, heck you could add a number of guys to help (Watt, Suh, Vernon Davis, Graham, Tillman, the list goes on). But I think the 1st 2 guys would be the most ideal for how Denver is set up right now.