OP didn't say who had the best career ever. He just asked who was the best defensive player ever. Therefore, Lord Watt is easily the best ever. Nobody has even come close to making as big as an impact on the field as Watt on the defensive side of the ball. If this said who had the best career ever then it would be Reggie White. But if I had to select one person in their prime on D it would be Watt.
/thread
Watt 6 seasons in, 83 games, 76 sacks, FF 15, Tackles 299,
White 6 seasons in, 88 games, 95 sacks, FF 13, Tackles 613