User blog comment:Neowitch/Place your bets, folks!/@comment-188.177.149.50-20120517214119/@comment-188.177.149.50-20120517215603

My answer to the question is Big Mom. Luffy has a grudge against her and I think Oda presented this thought because Luffys crew defeating a yonkous crew alone is highly unlikely. So to make it more obtainable Law suddenly wants to take down a yonkou.