The defensive back position is a difficult one to rank, as so many different players at this division play this position very well.
The two different positions for a defensive back are cornerback and safety.
Cornerbacks cover the receiver so they can't catch the ball, and try to intercept the ball, while the safety plays in the backfield, doing what their title says, being a "safety."
We have seen cornerbacks have a big year one season, then completely fall down the list the next (for example, Antonio Cromartie).
It seems as though the best safeties around the league are the ones who are often injured, for example Ed Reed, Bob Sanders and Troy Polamalu.
Although, the best defensive backs aren't shown through stats, but through their play on the field.
I expect some controversy here, but I feel as though I did the best with the current top defensive backs in the NFL.