Atlas - North Blue
It is not clear how weak or strong North Blue is actually. But since East Blue is the weakest of the four Blue's, it's clear that North Blue is stronger then this one. The little amount of North Blue places that have been shown or mentioned so far are listed below.
Places already visited by a Straw Hat Pirate:
So far there's no place been introduced that was visited by any of the Straw Hat Pirates. However Sanji himself said that he comes from North Blue so it is clear that there must be at least one place he's been to there. That's all the information that exists about it for now. It that should change at some point, it'll be mentioned here.