In the entire series FFX is my least favorite game by a pretty wide margin. But the question is about which ones are the "most popular," right?

Which game would win a vote would depend on the demographics you asked. But across everyone who has ever played a Final Fantasy game in the "West," I would have to guess it would win, simply because so many more people played video games by the time X came out than before that, and the ones after it were after the series had somewhat lost its mainstream "shine."