For KH to be competition, it would have to release a main series game more than once every 8 years, or however long it's been I haven't liked a FF since XII (or Tactics A2 if we're including those), so if KH3 is anything other than a myth I might jump on that bandwagon and vote for iT. As for now, FF
Well there wouldn't be a Kingdom Hearts without Final Fantasy so I think the correct answer is obviously FF.
KH became too convoluted with all the side games.