With the exception of XII, Tactics, and maybe VI, this game has the best writing in the series by far. It has a lot of subtlety than many other games in the series (X, for example) lacks, and a lot of the dialogue is laced with double entendre and metaphor that gives it an edge over the other games. In particular, I found the game funny and charming when it needed to be, but also serious and downright dark without seeming forced.
So, let's talk about IX's great writing.