Except that they more or less did. They just avoided using the word 'best' by using the word 'complete' and to a lesser extent 'polished'.
Being the "best" isn't necessary being the most "polished" though as some have pointed out. For example FFVII is a FF, I would consider the most polished in series is as in it has lot's of content, fairly complex mechanics, interesting worldview, but has other aspects such as it's plot (which starts of strong), interchangeable characters, coherency etc that weaken it. Basically, "polish" here vaguely defined.
The fact that the same guy is making the same promise that he made last time about something that wound up being so awful it felt like it was pulled from some Deviant Art's web comic simply doesn't bode well for this game.
I don't remember any other times when he said this game will be the "best. I know he's hyped some details of his game, but we know pretty every developer does this.
Nevermind that the brand has been bereft of a good game since XI (or earlier depending on who you ask), this isn't even a flagship title which have a history of turning out poorly, and the fact that the games that this spun off of didn't work and rather than admitting defeat and starting over with a fresh new approach they've been persistently pushing the same setting, the same
characters cardboard cutouts, the same deus ex machina prone to tossing out 'get out of jail free' cards, and so on.
For all intents and purposes Lightning Returns existentially that, a new setting that takes place in same world with different gameplay and mostly likely new characters. It's about as fresh you can be working on a sequel outside having completely new cast. Yes it's true that mostly FF spin-off are generally lackluster, but doesn't mean it haven't been goods ones that can compete with main series. Many would argue that FFT, FFX-2, Type-0 and Theaterhythm are better than some of mainline games. I don't Toriyama needs to admit anything or can anyway. Just improve and make a good game as expected of any other FF.
BTW, attributing "questionable storytelling" to just FFXIII or Toriyama is pointless, FF (and RPGs in general) are full of DEM. FF has those problems every game regardless of his involvement, not even FFVI is exempt from this. The only difference is that some games handle it better than others, Toriyama certainly did so in FFX.