The best video game I have ever played. Period.
First off are the characters, each character is well defined and given their own special niche for battles. Auron is the strong one, Lulu uses black magic, Yuna uses white magic, and Wakka can hit flying enemies.
Rikku, Tidus, and Kimahri have their own unique talents as well.
Next is the plot. A very interesting storyline, I will admit, that does seem to drag on, but that's the purpose of it. For example, in the middle of the game you lose Yuna for a good while, forcing you not to build her up.
But the absolute best part of this game is ..
The Battles. Never have I enjoyed playing a RPG this much. Some battles require lots of thought, planning, strategic moves and attacks, and switching characters. The ability to switch characters in the middle of a battle is what makes the game. If you are fighting a flying enemy, bring in Wakka, if the enemy is vulnerable to magic, bring in Lulu. And the best part is with each battle, you build up your characters. Learning new abilities and gaining strength, by the end of the game you will be doing massive damage with just one physical attack.
Overall, the game flows perfeclty, the battles are not too easy but not too hard either, the characters are so well defined, the ability to summon aeons is a great addition, switching charcters in the middle of battle, gaining strength as the game goes on, the list is endless..
Even if you aren't a RPG fan, which I am not, FFX is defintley worth the playing time, especiallty since its only $20 now.