User blog comment:SilverHexxitFights/Which is Terraria's Hardest Boss?/@comment-24029122-20131115010636

The Twins are both the best designed and the hardest boss. It is a formidable fight and with the same gear you destroy the Destroyer or Skeletron Prime you're gonna have trouble with the Twins. It is definitely harder than Plantera if your strategy is good, since Plantera is a situational boss, while the Twins, you can fight them everywhere. Of course, once you get Spectre Armor all of them become as easy as pie, but I think that's not the point here.