Atlas - South Blue
It is not clear how weak or strong South Blue is actually. But since East Blue is the weakest of the four Blue's, it's clear that South Blue is stronger then this one. The little amount of South 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. It that should change at some point, it'll be mentioned here.