Well, the one that I've replayed the most is 10 (I blame blitzball), however I enjoyed 7,8, 9 & 12 also. 13 I should probably try to finish some day, and I have 6, but have yet to play it....
In my opinion it is final fantasy six the characters were just so epicly awesome specially Sabin and Setzer for me they are both lvl 100 and kick major face with attacks of 9999 and ultima spells