As of now...
Final Fantasy Mystic Quest - Keep losing my interest, then I come back to it and forget what I was doing.
World of Final Fantasy - Started and just didn't really gel as well as I had hoped.
Final Fantasy Crystal Chronicles - Started at a friends, but I've never owned it. May pick up the new version to fix that.
Final Fantasy XI - Started but just never got hooked enough to dedicate any real time to it. I may attempt to play it again soon.
Final Fantasy: 4 Heroes of Light - Literally at the last dungeon and got smacked with a big difficulty spike that I don't feel like brute forcing my way through.
Final Fantasy IV: The After Years - Got to the final dungeon and lost interest. I have been thinking of trying to restart it though.