First fully completed game was FFVI and continued from there. I loved them all the way to XI, when it went online and I played for 2-3 years. Didn't like how they changed the combat in XII, and thus only played a few hours of that and haven't played any of the new ones. Also, still not sure how I feel about FFXIV being a revamp of XI. FF:CC was different, and a lot of fun. Dissidia, and Dissidia 012 were great as well, just because I friggin' love fighting games. Basically,
Final Fantasy is great, and dang, I need to replay them!
started with VII but it was a borrowed copy. first fully beaten was XII. I'm restarting VII for the billionth time because every time I get my hands on it something happens and I cannot finish the game. this time will be different.
It's a tie between Final Fantasy Origins and Final Fantasy VII, since I got them both at the same time about six years ago. I'm still grateful that I got the two at once, since they're each an introduction to a Final Fantasy generation, in a sense. I believe that if I got one, but not the other, I wouldn't appreciate the wholeness of the franchise today.