It is often said in the security community that to be the best one must be the best attacker. administrator hacker defender social engineer

The answer is defender because in order for a security community to defend themselves or to gain the position of being the best defender, they should have the ability of being the best attacker for one couldn't just shield themselves from harm because in order to protect themselves, they must fight back or attack. Because defending themselves wouldn't improve or gain any changes as they try to win.