Once Rashaan Melvin decided to sign with Oakland, the Colts immediately lost their best defensive back. Melvin was a lock down corner who could play most receivers one on one without much help. That opened up everything else for the defense.
With him no longer in Indy, it becomes difficult to decide who the best defensive back is on the roster. Everyone in the defensive backfield is extremely young, and not many players have had quality playing time to prove their worth.
Malik Hooker has the best chance to become the best player.