Once upon a time, my answer would have been Parcells, but after witnessing his stop in Dallas, I would now be forced to say Phillips. My decision would not favor Parcells because the NFL did not outgrow him, but general player attitudes have.
These are two different types of defensive coordinators. Both coaches teach their players to be smart and execute their assignments, but what separates both men is that Parcells demands his players to also play through all physical obstacles like pain and injuries. He would mold his players not unlike an Army drill sergeant would.
If a Parcells defensive player were asked to run through a wall, they would probably do so without hesitation--while a Phillips defensive player might ask first what the wall was made of or how thick it was. In years gone by, Parcells could pry that sheer level of determination out of a defensive player such as Willie McGinest, Ted Johnson, Otis Smith, Mo Lewis, Jason Ferguson, Jim Burt, Leonard Marshall, Pepper Johnson, Carl Banks, Harry Carson, Elvis Patterson, Lawrence Taylor, etc., etc. Nowadays? Not as much.
I played high school football for a coach with the same mindset as Parcells who demanded the same level of commitment from his players. I watched some players wilt under that kind of pressure. Some people just can't take it or are willing to take it. Their morale shrinks. Some didn't play as hard as they could have. Others simply quit altogether. Still, if a player can endure that kind of pressure and thrive because of it, they will succeed by taking their game to level higher than that of their opponent. I just wished my coach had been a lot smarter in the X's and O's department also.
It's a mindset. It's sheer willpower. It's hard-nosed, down-and-dirty, football at its best. It's the Parcells way, but it's no longer necessary in today's NFL. If your defensive players are smart enough and execute their plays more often than the opposing offense does theirs, it's more likely your defense will be dominant enough. Parcells' coaching style simply enters an additional x-factor into that equation for an added advantage.
So, as I said earlier, with today's players, I would have to say B.) Phillips. Final answer.