My opinion is that it's generally been in "decline" after a steady upswing and peak at VI and VII (with V being the climax of the old-school portion of the series). However, to me, the noticeable decline didn't begin until the PS2-era.

VIII is an interesting experiment but falls a little flat with an easily-manipulated (but wonderfully inventive) customization system and an unfocused plot.

IX is a solid, better put-together game than VIII, but ultimately felt a bit beenthere-donethat and completely struck out trying to fuse an old-school FF story with a PSOne-era one. Still a solid game - I'd take it over almost all the NES/SNES games for which its designed to provide warm, pinky nostalgia - although I don't find it as interesting as VIII.

Except for a few bits, X is a complete lame duck both in gameplay and story. Square actually used the PS2 technology and made the series feel disappointingly smaller and thinner. As an 18-year old who, up to that point, idolized the series, X was the first time I thought, 'Wow, this actually sucks.'

XII might be one of, if not the, best game with Square's name on it. However, for better or for worse, this game seems to be Final Fantasy in name only. I usually consider it apart from the main series progression (and any discussions of decline or peaking), especially after the way in which XIII felt like it was designed as a progression straight off X.

XIII - for me, the real FFXI, as it seems to pick up where X left off, abandoning XII (and XI Online) as an anomaly. XIII is easily the weakest game with the Final Fantasy name on it, and one of the most pointless games I've played in a long, long time. Outside of battling, there is nothing. The story is trite and poorly-written, and I look down on anyone who thinks otherwise. The gameplay might have been interesting, but even an incredible, groundbreaking battle system (ie: not XIII's) couldn't carry a game for the amount of time SE expects the player to spend with it - and definitely not without a living, breathing world surrounding it in which the player can get lost, or in which he can find pleasant diversions and curiosities (SE used to be really good at the world-building thing).