I always loved the premise of Final Fantasy Tactics (and LOVED it's many plot twists), but it is hard to break Final Fantasy VI's endless characters and dynamic storyline. I never cared for much of Final Fantasy VIII's story, but it's use of the themes of love, loneliness, and friendship is stand-alone in the series.

When it comes to characte examination and in-depth history/story, Final Fantasy VII tops the list. As faras addictive gameplay is concerned, I still love taking out my original Final Fantasy (though my NES works less and less these days).

It really depends on my mood at the time, since they are all awesome games. I guess it's a tie between VI and VII then.