By saying best CB, I would say the most important thing is consistency, so I would suggest Phil Jones. Strong, quick, consistent. He can even fit in 3 defenders formation. My save is 5th season and City bought him for 27.5m, his ratings were all above 7.2 for the previous seasons.
Chiellini is the best. Also recommend Piqué but he doesn't want to leave Barca, does he? Also Mamadou Sakho, Simon Kjaer, Raphael Varane, Nicolas Otamendi, Neven Subotic, Mats Hummels, Benedikt Howedes are beasts too.