User blog comment:IamJakuhoRaikoben/Sorcerer Magazine, Issue 50: February 2016/@comment-28052271-20160325093621

I think that it would be Mard Geer. It took a lot more mages to defeat him than Hades. Right? Not to mention that the mages that fought him were all pretty strong at the point of time, whereas when Hades was defeated, Gray and Natsu weren't nearly as strong as they were when they fought Mard Geer.