The honest answer should be Jürgen Klopp !!
But in terms of players, I would say Virgil, Alisson and Suarez. I was actually in disbelief given the season we had and were having, that the groundwork for Suarez was laid and almost done, with you-know-who still our manager!
Virgil quite simply bcos City, Scum and Chelsea wanted to sign him, and there were rumors that Barca considered him before they opted to wait until the summer to activate Lenglet's release clause.
And that was possible, only cos of Jürgen!