I dont think you can judge him not being the best because some games he wasnt perfect. Did Rio or Ramos or Terry always play perfect in every away game?
As far as a Klopp teams being a defensive machine, yes and no. When we had Lovren and Sakho this defensive machine didnt work so well.
Aside from his talent he is also a great organiser and communicator, he commands our defense. This above all else is whats makes him such a good defender.
And in terms of defensive ability last night showcased this when Son burned past Van Dijk and Matip he looked through on goal, Van Dijk put on the afterburners and just took the ball from him like he was taking candy from a baby.
Van Dijk has everything in the locker, power, pace, strength, acceleration, tackling, heading, speed, passing, leadership, communication, organisation, occasional goals.