Cahill is not our best CB, in fact he is the worst. Ivanovic, Terry and Luiz are ahead of him, Actually I saw one superb match by him, and it was against Barcelona. He was also really, really good against Bayern. I said before, I noticed this guy makes a lot of mistakes, his position is usually wrong, he easily gets dribbled by the opponents and its not uncommon to see him in panic inside the box. Luiz have 2 problems: sometimes he goes to attack and just dont return, and sometimes his decisions are not the best, like trying to dribble when he has to kick the ball far away, Terry problem is that he is old and is getting really slow and in the air he is usually beaten by the opponents. But in my opinion Terry+Ivanovic is our most "safe" defence, and Terry+Luiz is the most exciting. Luiz+Cahill means: no clean sheet today.