In my mind, the most unbeatable defense is what I've always called "The Alabama Stack".
Line up all 11 players one behind the other, starting at NG. Then, just before the opposing QB calls for the hike, our defense spreads out in a pattern known only to our players. That could be by height, or by jersey number, or alphabetically. It would toooootally confuse the QB because he would have no clue what was coming.
40% of the time, it works 100% of the time.