FF9 was a decline from the previous 3, I felt; the beginning of the series decline. It did have Treno, which was a fairly cool town. Everything was more simplistic than 8, the plot, the art, the gameplay, the characters, the world, the extras, etc... 8 is still the most complex of the series, and with the most to do.
FF10 had better characters than 9, better design; I thought the battle system was excellent. The towns were really bland though, very empty feeling. The world was also very corridor feeling. This is the game where it started to feel that technology was having an impact on creativity.
FF12 - great battle system, WAY too much fighting. World was nice, towns much improved over FF10, but a lot of emptiness. The story started nice, but then became very sparsly delivered.
FF13 - I like the characters, but as I said, the lack of real towns left me with a big empty space where satisfaction normally is when I play FF games. Graphics were very pretty, and it was very cool that they made one world very artificial feeling, and the other very organic. Too many battles. Encounter system felt like Mystic Quest with more animations until Pulse.
Overall, the newer games have improved in some areas; graphics quality (as a natural tech progression) and battle systems. They have also been declining sharply in others such as towns, plot, and the world. FF12 seemed like a failed attempt at creating a huge FF-like world that fit in with the new tech, if they would have let it succeed, it would have probably been the best. Nowadays, I fear Square has lost too much of their original talent.
-Insert Inspiring Quote-