The importance of protecting the QB's backside with a quality left tackle can't be understated. Conversely, getting to a QB with a terrorizing DE or LB can change the entire course of a game. Lets not forget the confidence a reliable RB can give you, and those primadonna WR's can certainly put the fear in an opposing defensive coordinator.
For me, there's nothing like a completely dominant DE rushing the opposing QB or a stonewall LT, but I have to go with a monster Defensive Tackle. The right DT can setup the rest of a defense. That's particularly true in the 3-4 defense and goes without saying you need two DT's in a 4-3 defensive scheme. Know thinking about it, maybe the long-snapper position doesn't have quite the same importance as I may have assumed. There's no denying the fact that the success of an NFL team depends on every team member, but which position is more important?