Barcelona - Mats Hummels
Barcelona have been utterly outstanding in La Liga this season, scoring 34 goals and conceding just seven.
They're well on their way to another Spanish title but there are definitely some gaping holes becoming apparent in the defence.
With Carles Puyol in the twilight of his career, he'll need replacing and Mats Hummels has been linked with the role.
With Bayern Munich throwing the gauntlet down last year in the Champions League, Barcelona need to strengthen to challenge in Europe's premier competition.
Hummels is the perfect place to start.