FFIV: Rydia and the plot.
FFVI: The plot was amazing, most of the characters were great and well developed (Terra being my favorite, obviously), and Kefka was hilarious
FFVII: The plot, Aeris, great music, and the characters
FFVIII: Love story, plot, and great music
FFIX: The comedy, plot, great music, and funny characters
FFX: Rikku, Auron, and the graphics
FFX-2: Non-linear game play, dresspheres, the music, and Paine's background was really well developed, especially considering that she wasn't in the first one.