Final Fantasy X was my first venture into the series, then Final Fantasy X-2 when I learned that there was a sequal, and now it appears we have another winner in Final Fantasy XII which in my opinion has actually made some improvements over Final Fantasy X. For starters the battle system, which was more like a chess game in Final Fantasy X,, is more action based and realistic now; it's not quite the imediate real time system that Kingdom Hearts uses as there is still some delay between entering a command and the actual execution, but it is closer. And even the delay is not such a bad thing because it gives you a brief amount of time to change your commands if you decide you don't like what you see. Also, after a certain point you can preset comands called gambits which characters will follow instinctively on the battlefield.
Secondly, the sphere grid that I praised in Final Fantasy X returns here, but in a slightly different fashion. In the previous game, characters earned experience by winning battles which then earned them spheres, and they each started on their own section of the grid. It was far more difficult for characters to learn each other's abilities. Here wining battles earns characters experience points which are used for traditional leveling up of health and magic, and license points which they can take to the license board and spend to obtain the right to use various equipment, abilities, and status improvements. Further, you don't just inherantly learn abilities once you obtain the license. You have to purchase them from merchants.
My third point is the elements of the game: there is the main story which covers a world far more expansive than that of Final Fantasy X, there are side quests that you can unlock as you explore more places and speak to more people (a requirement of any Final Fantasy fanatic), and something unique to Final Fantasy XII--hunts. These are a series of optional bosses that players can fight in order to earn more money, items, and improved rankings in a special hunters' clan.
Finally, the game offers a wide variety of magic spells. Summon spells, which were absent from Final Fantasy X-2, have returned here in the form of Espers. And while only Lady Yuna could summon Aeons in Final Fantasy X, any character can summon once they obtain the license to one Esper. In addition, characters can obtain the licenses to Quickenings--very powerfull magic spells that come in quite handy in lengthy boss fights.
I've spent so much time praising Final Fantasy XII, but the game does have some flaws. Chief among which is that you have to constantly readjust the camera because it refuses to stay put. Also there is one glaring incomsistancy. Experience points can only be obtained by the charactrs who were on the frontline when a fiend is defeated. License points, however, are awarded to all members of the party that are not knocked out. Even if they were not part of the party when the battle took place, they still receive license points for that battle. But inspite of these set backs the game is very well thought out and highly recomended.