Ah, finally an easy question with an easy answer: get Final Fantasy X It has its problems (many of which were covered extensively by Spoony), but it's still quite possibly my favorite game in the franchise. It sports great music, easily one of the best Final Fantasy stories, and an interesting character-growth system in the Sphere Grid. It also managed to inexplicably make turn-based combat interesting to me again (along with Personas 3 and 4), and I am not usually a fan of turn-based combat. As silly as the world is when you get down to it, it also sports incredible art design (this game still looks pretty good) and is remarkably consistent with everything at some point or another based on circular concepts. Sure, Blitzball sucks and makes no sense, but at least you can ignore it outside a couple of story beats.
Then you have Final Fantasy XII, one of the most disappointing games I've ever played. Throughout the game, it tries to be big and epic with a focus on backroom political machinations...which turn out inevitably to go absolutely nowhere. There are at least two characters (the two characters presented as the main characters at the start of the game, mind you) who are completely superfluous to the game, and the rest of the characters fare no better. The only worthwhile character, really, is the Han Solo-esque Balthier but even he can't save a game this boring. The combat system is incredibly dull and reminds me of an MMO, as do the rest of the quests in this game. The art direction is fairly bland as well, especially compared to FF X.
Ideally, at some point Square-Enix will finally get off its ass and release an HD collection with X; X-2; and XII, but until then stick to playing X.
And by the way, if you end up liking X don't play X-2. IMO, X-2 is a very weak game that rips the heart out of X to fashion a narrative that is nowhere near as interesting.