The series peaked at the back-to-back masterpieces of VI and VII. VIII is one of the worst games in the series with its slipshod narrative, terribly written cast, and gameplay that can be broken with no effort whatsoever. It was no wonder that Sakaguchi yanked the reins of the series away from VIII's dev team and gave it someone else.
IX, unfortunately, still wasn't quite there. The battle system was back to basics and fine and the characters were likable, but there were a few problems. The plot, for instance, was kind of meandering and loose, leading to confusion at times. The skills system was also a bit clunky and led to a lot of instances of grinding for abilities. A good game, but a flawed one to say the least.
X had a more coherent narrative and by far the best battle system of the entire series. The supporting was interesting as well as Tidus but it suffered from two big problems. 1) Yuna was BORING. Seriously, she was a stiff, uninteresting heroine and because of that Tidus was left with a love interest that killed his development. 2) Seymour might very well be the worst villain/rival in FF history. He was annoying, boring, and uninteresting. I know Square was drained after Kefka and Sephiroth, but a terrible, uncharismatic villain leads for bad conflict. Again, a flawed game but still good.
XII is where things officially turned BAD. Before Sakaguchi left he hand-picked the game's director: Yasumi Matsuno, a man whose track record is absolutely spotless containing some of the best narratives in gaming. However, the team consisted of the FF Tactics team and the FF IX team. Long story short, the FFIX team threw a hissy fit and Matsuno left halfway through development due to the amount of bullshit and it shows in the game. The battle system is painfully boring, the characters the worst since VIII (three of them having NO justification for their existence), a storyline that is impenetrable, and a stupid skills system. That was the point where FF died and Square has merely been directing a desecrated corpse.
As for XIII, I needn't say anything. I would say that Square has a chance to save the series if they handed the reins to Matsuno, but considering the amount of bullshit he had to stomach the first time I doubt he'll ever touch the FF series again. Best to just euthanize the series at this point.
Dear Buddha, that's a lot of words.