keep Agger, Fazio and Otamendi, sell Skertl (he is not that great at all)
if you want to sell another, i would suggest maybe Agger then or even Otamendi and replace them with some of the following:
Try some of these:
Juan Forlin - Espanyol
Bruno Alves - Porto
David Luiz - Benfica
Nevan Subotic - Dortmund
Per Mertesacker - Werder Bremen
Breno - Bayern
Sidnei - Benfica (good young defender and really cheap)
Alex Silva - HSV
Naldo - Werder Bremen
Lukas Sinkiewicz - Leverkusen
Serdar Tasci - Stuttgart
Benedikt Howedes - Schalke
Ivan Marcano - Villareal
Federico Fazio - Sevilla
Diego Lugano - Fenerbahce
Allessandro Gamberini - Fiorentina
Felipe - Udinese
Christian Zapata - Udinese
Geromel - Koln (best cover defender in the game, works well with Yobo too)
Elo mate, Otamendi is my best defender, I signed him for 3million in first season and he was my best defender and was voted "best signing" that year too.
If you had to choose maybe... 3 or 4 out of that list, who would you choose?