You know what defenders I like best, not behemoths like boulharouz, vidic or even chiellini(I love him though) I think the best defenders are the ones who spend a lot of their time pointing to where others should be, but at the same time aren't afraid to put in a very neatly timed sliding tackle when it necassary.
So that's what I hold as the most important attributes in a leading cb, anticipation and positional sense. If you put canna in his prime next to the chiellini of today, that would be most certainly the best balanced back 2 in the world.