FF6 was the best game in the series? No. FF7 by far. FF6 had shallow characters and a generic storyline that didn't even try to go deeper than "save the world from evil empire". It wasn't a bad game, it was just the STANDARD for RPGs at the time. It also had very a very basic system for character improvement that involved little or no ingenuity, as opposed to, say, Materia, Junction, Ability learning, the Sphere-Grid/Chrystarium/Weapon "leveling", and the License Board. FF7 made RPGs into something far more involved.
As for FFXIII, one of the few legitimate complaints Yahtzee made (from a critic standpoint; his hatred for the characters only acts as humor) was that the game should've been good from the start, and not taken a while to get momentum. Many games are like this, but FFXIII had an especially long "tutorial" period. Nevertheless, had he actually played for longer, he (or rather, fact) would've attested to vast improvements in the storyline and gameplay. Complaining about the storyline and the characters lacks substance when you've only spent five hours with the game. After all, no good story allows you to know everything you want to at the beginning. Still, the review was amusing.