Sokratis was BVB`s best defender last season. Hummels being better overall can be thoroughly argued, but this is no clear cut case.
BVB is an elite club, right outside the group of the Bayerns, Reals and Barcelonas, etc. which have a squad of 11 + world class players. An elite club, on their end, has a few world class players surrounded by elites. Dortmund has three world class, one in each third of the field: Aubameyang, Reus and Sokratis. Then they have about 6 or 7 elite players complementing them.