I think, when looking at their defence, it's kind of relative to the rest of the squad.
Given the attacking strengths they have, in comparison their defence looks quite weak, but, they have conceded fewer than anyone else.
As has been shown in their last few games there are ways through, and a lot has to be said for the way teams set up against them, BUT they have had the 'most effective' defence so far this season - can't be argued when you look at the Goals Against column.
They have scored more than any other team so far, so the attack is working. they have conceded fewer than anyone else so the defence must be working.
I agree though with the main point, there has to be a consistency for them to be called 'best ever' - Arsenal's 49-games unbeaten (don't need to be reminded where that run ended!), Liverpool, United etc can all have a greater claim for that title, but with a summer window I can't see City getting 'weaker' if I am honest, and it will be up to others to catch up or catch on.