Atlas - East Blue
Going by what Mihawk said when he first met the Straw Hat Pirates East Blue is the weakest of the four Blue´s. And it is the one of them about which has been shown the most so far. This is of course because the story starts there. All of the places that have been shown of East Blue are listed below. Those that have been visited by the any of the Straw Hats and those that weren´t so far.
Places already visited by a Straw Hat Pirate:
Since Luffy himself comes from East Blue and meets Zoro, Nami, Usopp and Sanji here, there are some places in East Blue that were visited by a Straw Hat Pirate. They list as follows: