From a technical standpoint, I don't think that any of the FFs have "great writing". That being said, the immersive value of the stories are most often superb. Obviously there are issues with translation as far as dialogue is concerned. But, what's important to note is that the story-telling capability of a video game is not based solely upon the words that you read. It's the same as in a movie--you don't find yourself embroiled within the story because of dialogue alone. Now, the dialogue has to be halfway decent, but the acting and the direction also play a pivotal role in creating the story. Music is also a huge part of it.

Now, there are some serious golden moments. In FF7, for example, I think Cloud speaking to Cloud (the black-screen freak-out moments) are very well-written. You can hear a sombre, caring voice speaking to the MC just from reading the text. Specifically, I mean the scene in lower-Junon after you save Priscilla.

It's really a tough question, though, because it's nearly impossible to actually pull away the dialogue from the medium. Still, considering you can watch a movie and think "That was some killer dialogue", then I guess none of the FFs are really all that well-written, but the direction, and all the other elements combined turn them into very interesting stories to follow. Regardless of whether the dialogue is "Good", all the elements work together quite well for the most part.

The bad moments, of which there are countless, to be honest, are the same problem you get with a lot of sci fi, where the idea is bigger than the writer/artist's ability to convey that message. Even the best Phillip K. Dick books suffer from spots of awful writing. And that's coming from a PKD fan.