Wow. Whoops. Clicked submit by mistake. Lol. Roy Mustang is an option as well.
@taichokage: I agree with Natsu only being able to absorb a certain amount of flame before getting overwhelmed by Ace. The other thing with One Piece characters is that they never seem to run out whatever power they control so I think Ace would just out flame Natsu.
Ace and Natsu can manipulate their own flames, but Roy should be able to redirect other sources of flame since his itself is not even his own but merely growing a tiny spark of fire into an inferno by guiding oxygen into it and such. I'm not saying he can beat them, but only replying to a comment that he can't manipulate fire. Actually he can only manipulate it not create it since he needs that spark to start it.
I just don't understand why mustang is being undermined here, he is foremost an alchemist before a flame manipulator and thus need not solely rely on it. Also we know he can transmute air and without oxygen there is no fire. All he has to do is pull the oxygen away from their flames does he not, therefore 'killing' it? Though physically they are both superior his flame manipulation cannot be overlooked.