Mine is Final Fantasy Tactics for the playstation.
Great storyline, solid gameplay, it was hard and the playabillity factor was pretty high.
Tactics is definately the best and my favorite aswell. I loved the gameplay for this one. I suggest you pick up Tactics Ogre if you liked FFT's battle system, its the same style.
What is it for?
I tried Final Fantasy Tactics Advance, it just wasn't the same thing, I liked the PS one way better.
Not to mention the storyline on the advance was confusing.