I don't think where he ranks in terms of signings matters.
We needed someone to lead and organise the defence, we bought him for a fortune and he's performed well so far.
While he's been the catalyst for our defensive improvement, we've also seen the signing of a world class keeper plus Trent and Robbo with a years extra experience, the emergence of Gomez at Centre back. On top of this we've seen the evolution towards a less risky style of play and have bought a specialist DM in Fabinho.
I think all of the above was necessary to get us competing for the league. Even with VVD on board Klopp saw the need to evolve both the personnel and the style from the team that got us to the CL final. Losing a 2 goal lead v West Brom away, conceding 2 late goals v Roma and then 4 in the away leg all pointed to the need to tighten things up if we wanted to challenge for the league.
I loved the 14 side and the early Klopp teams, but this is the one you'd trust more.