Last great FF was VII. It was innovative, but still remained faithful to the series. I'm not a big fan, but what came after this one is either boring, ridiculous, tedious, annoying or just not a FF game... or worse, a combination of all that.
VIII's story was kinda good but the game itself wasn't challenging. You could become Superman in the middle of disc 2 with all those junctions.
As for IX:
I agree. The first 2 discs are great, but then the plot goes down and crashes.
X's story is utterly ridiculous and Tidus is annoying as hell. Aside from Auron, Lulu and the combat system this FF has nothing worth mentioning. I hated blitzball and still can't understand the thought process that ended in the conclusion this minigame is fun.
X-2 is plain fanservice. No plot, no sense whatsoever.
Never played XI, since it's a MMORG and there are better ones. XII is good, but it's not a FF. Looked and played more like Vagrant Story (which is no coincidence, since the same team that made the latter worked on this one). The story was very intricate and I thought it was interesting to see a more politically-focused plot, but the game itself becomes tedious very fast.
I'm currently playing XIII, 'cause I can't spend countless hours playing anymore and so far (I'm starting disc 2) I find the story interesting, a little weird, but it stands. Vanille is annoying, Hope is even worse (too bad he has the best magical powers), Lightning is female Squall (enough said), Snow tries too hard to be a "hero" and the only really interesting character I've found so far is Sazh. He seems more human than the rest of them and is definitely more likeable.
I understand Hope, really, but some times his attitude (especially when it comes to Snow) just annoys me. I mean, tell the guy already! Or better yet, tell him about your mother and then punch him in the face, he'll feel so guilty that he won't hit you back. Buuuuut I guess, they have to make this conflict go on for as long as they can to add to the plot.
It's a shame, really, the great FF classics had better plots, more likeable characters and a good combat system that made the experience enjoyable. Lately, the developers seem more worried about making incredible CGs and coming up with the most complicated plots and forget about basic things such as who is the protagonist of the story. In FFXII, it's not Vaan or Penelo, it's Ashe. In X, it's Yuna. XIII has found what I think had been missing for a while, a balance between the importance of each character. I still have a way to go, but so far, I've seen the story focus a little on each of them, except Vanille. I know there's a reason for this and I have my suspicions, but I'm playing along.
I really hope the next game becomes the last great FF, because fans deserve to have another title as epic as FF6.