Gameplay wise, XIII is the strongest game in the series. You cannot fucking argue this. The Job system has been re-worked and refined as the Role system, where many of the series' staple moves and abilities have been modified and simplified to fit six basic "jobs", all of which are actually useful in battle. Characters can input up to six commands at a time, and can act before the gauge is full, making combat flow much better and faster. MP is gone: everything uses varying amounts of the ATB gauge. Awesome.
The chain system, lifted from Final Fantasy X-2, has been re-worked into a system that heavily impacts how fights play out. Chaining and Staggering determines how much damage you deal to enemies: the higher the combo chain, the more damage you deal. When enemies hit a certain point in the chain, (it varies by enemy type), the enemy will enter "stagger" state, making it vulnerable to damned near everything, and dropping it's defense to shit, leaving them open to all kinds of sodomy.
All in all, the gameplay is a fucking model for future games in the series.
So why don't I like it?
Well, the characters are shit, the story is shit, and worst of all, the developers prioritized the story. The first five hours of the game has you running around linear corridors doing all sorts of action-movie-esque stupid nonsense, all the while the game is throwing out random fucking jargon information that doesn't make sense to anyone who isn't a developer or has a fucking summary of the story in-hand.
Once the developers pull their heads out of their ass and start telling the story, you still continue playing the game on rails for hours on end before the game actually lets you do something that isn't story related. I clocked in at about 22 hours before Grand Pulse (where the game opens up). Twenty-two motherfucking HOURS. That's a long fucking exposition for a game with such a simplistic, melodrama-riddled story. What does the story boil down to, that requires such a long and elaborate set-up? People fighting their fate. Defiance. Not accepting the circumstances, and fighting to survive. Thats IT. Square-Enix took over twenty hours to say that. Jesus weeping on the cross.
I hate Final Fantasy VIII for similar reasons. The gameplay improved in every way from Final Fantasy VII, but the story was such angsty shit, and some of the situations the characters found themselves in were so fucking ridiculous, I actually couldn't play past the first disk without feeling my gag-reflexes kick in.