I think X-2 is where things began to change for the worst. I liked playing that game, but it's more of a glitzy gaiden game than a sequel and it misses the sort of mood that Spira previously built up. It's a game of reused assets and uninspired, copy-pasted add-on dungeons, and the music ranges from listenable to forgettable and pretty much always misses the mark that X set. Even as a different, more peppy style, it feels a bit forced and generic.
I mean nothing could trump FF9, in a way that's the last great FF game. It was the last game by the series creator, it was the culmination of the Playstation trilogy and it was the last game with an expansive, no-holds-barred script - ever since, the games have had to work within the confines of voice acting, an that has crippled what the games can get away with.