this is part of an interview with kishimoto
SJ: While we're at it... could you please tell us if Minato was the strongest shinobi that ever lived?
MK: Hmmm, it's a tricky question. As far as dead shinobis goes, then yes, he's the strongest. I'd say that Sandaime shares the number one spot with him. As far as living shinobis goes... well, without going into many details I'd have to say no. There's a couple of shinobis whose stronger than him.