Thread:Blade0886/@comment-31259708-20160610073628

http://www.kingdomhearts3.net/dengeki-magazine-article/

"as a result, Sephiroth, who was thought to be the strongest in Kingdom Hearts II is now lower than the Organization members in terms of ranking from the strongest." - quote from nomura him self

So from this it seem's like Sephiroth was the strongest being in KH until Orginization 13  was a thing  