In order from fewest to the most completion)

Never played II, III, X and XII

V - Lol always stopped middle of the game, not sure why.
IV - same as V
IX - same as IV, although I am in the process of beating it this time
I - Once. Will get back to this once I get a DS
Tactics - Twice
VII - Twice/Thrice
VI - More than 3 times
FFTA - More than 5 (1 save game that reached 300 hours )
VIII - I lost count. Currently doing a perfect low level game

ps: My fav FF is not VIII but I just love to replay the game