6 is objectively the best.
And that's where you're wrong. It's a great game, but the problem with it is that it gets muddled at the halfway point. When the massive world changer happens, anything you do there has absolutely no direction and doesn't give the player much clue what to do next.
Now, it's also one of my favourite Square games, I even rate it higher than the much better paced and built Final Fantasy 4 (AKA 2, way back when... "YOU SPOONY BARD!"), but it's not the best in terms of game construction and pacing, that goes to the aforementioned FF4.