User blog comment:EvanDude345rv5/Fairy Tail 356: Tartarus Arc Prologue/@comment-7418318-20131020091019/@comment-5580383-20131025051719

Acnologia was holding back when he blew up the island and it was stated by Levy that he  once destroyed an entire nation,currently  it's Hyperbole considering we've seen no feats supporting that feat however considering  there are 10,000 dragons  that are all weaker than Acnologia are each capable of leveling mountains,  Acnologia being able to at least level a nation singlehandedly isn't that  far out in terms of hype.

Zeref is  lacking feats altogether when it comes to destroying the world and really we need to see how strong E.N.D is before we can hype his power or speculate his power  but considering that an unawakened Zeref one shotted Hades and Zancrow with no effort(ablet  both were weakened) and he's hyped to being as deadly as Oncologica, It's not impossible to see him at least stronger than Warod Seeken who has a decent 2,000 km tree feat recently.

Acnologia and Zeref would be somewhere between Current Blackbeard to  Juubi in terms of potential power.