I - The classes levelling up. General awesomeness.
II - The learning system. And Minwu.
III - Three airships!
IV - Five-member parties.
V - Job system and multiple worlds. The humor.
VI - The story, music, setting, characters and the way summons were used.
VII - The setting, the cast and Aerith's death. Also, 3D world map.
VIII - Squall, Junction and Ulti's castle.
IX - The nostalgia and references to older titles. The little gimmicks and fairy-tale atmosphere. Also, Vivi.
X - Some ways in which the battle system could be abused, despite it's slowness. The music.
X-2 - Jobs again. Slight non-linearity. Great combat.
XI - The races, classes (system in general) and the fact that my Elvaan warrior could co-operate with my girlfriend's Mithra black mage. Seriously, I loved dragging away the enemies so that she could do her magic
XII - Everything. Seriously, I love the setting, the art style, the music, the vastness of the world, the quests and freedom, the combat system, the grinding and most of all the hidden depths of the story. And the superb voice acting and localization.

Tactics - The PSP localization, the terrific story.
Tactics Advance - The races, the surprisingly deep story.
Tactics A2 - The mini-stories in quests (the main line sucked), the stream-lined job system, varied quests, loot.