I'd say you don't necessarily have to have a stellar quarterback, RB, or receivers, though your quarterback had better be able to manage the game, and they must work well as a unit avoid turnovers, and do things like pick-up blitzes.

I'd say without fail you need at least one stud LB, a decent cover corner, and at least one defensive lineman who commands double-teams.

As far as disparity between best and worst, I say CB. You can count the # of pure shutdown cornerbacks in the league on one hand. And there are others who are dead meat if you get them on an island against a Moss or a TO.