There's no right answer here. Everyone has to play WELL (not to be confused with above average or fantastic) to be dominant.
Let me break it down for you:
Corner:
Good corners: Good corners shut down their side of the field, making it easy for safeties to concentrate on stopping the run, short passing, and double teaming the #1 WR. They also get off their blocks and help in run coverage. (See Ashton Yobouty)
Bad corners: Bad corners get beat short, then short again, then miss a tackle, then get beat deep. They are inneffective in helping stop the run, making the safeties cheat up to help, meaning the deep ball is going to be open sooner than later.
Safety:
Good Safeties: Good safeties take away the home run ball. Good safeties recognize run early and are always around the ball when the play is on the ground. Good safeties study film, know when play action is coming, and play their assignments until it's safe to bail someone out. (See Nate Salley)
Bad Safeties: Bad safeties make it hard on the corners. They are late getting to the play, bite on the play-fake, and get blocked downfield by lineman on a screen play. It's called SAFETY for a reason, and bad safeties make it dangerous for the defense.
Linebackers:
Good Linebackers: Good linebackers are unblockable, versatile, and above all, SMART. They can rush the passer, stop the run, drop back in pass coverage and create turnovers. (See the entire class of 2005.)
Bad Linebackers: I guess watching games other than OSU games the past 8 years has helped out with this one, because if you watch OSU, and ONLY OSU, you don't know what a bad linebacker looks like. Bad linebackers are slow learners. They are late getting to the ball, ineffective rushing the quarterback, and to slow to cover anything more than a fullback in the passing defense. Bad linebackers get blown up on run plays, and get caught with their pants down in play action. Bad linebackers are breakfast, lunch and dinner to Troy Smith. (See everyone who plays LB at the University of Michigan, and this is single-handedly what's destroying them, let's keep this a secret.)
Defensive Line:
Good Defensive Lineman: Good Defensive lineman stuff the run, get to the passer on 3-step drops, 5-stop drops, 7-step drops, and shotgun formations. Good Defensive lineman are able to keep TE's at the line of scrimmage, get off the initial block, and get their hands up to tip/bat down passes.
Bad Defensive Lineman: Bad defensive lineman lose the time of possession battle every game. They don't get their hand ups, their center of gravity down, and are usually served with butter and syrup. They never get to the passer, and make Saturday afternoons and eternity for a defensive backfield.