Hrm... well, the best for me is easy: 5. It still had a story that was able to pull me in (though not to the degree of 6), whereas it fixed the class changing issues from 3 to a tee, and there was still enough challenge to make me have to be careful, which is how I can justify edging out 6 for the best.

Worst is a little tougher. 8's battle system sucked, the inability to remove what's-his-name from the party (tells you the memorability of the characters), more forced crappy mini games (same problem as 7), and in general, broken mechanics makes it a bad game. Or, there's 13, a painfully easy game with the longest tutorial ever, and a lack of any sort of exploration. And there's the issue where if the one character (why only one) that you control dies, it's game over. Like 8, it's got too many changes that are negative. I consider 10 very low because of its linearity and requiring character swapping in battle, but 8 and 13 are definitely worse. I may side with 13 being worse, though, since it's more of the direction things have been in lately...

FF1-4-6 are my favorites (nostalgia...)

13 is by far the worst in the series !!

Best one: FFX (It's also my fav game in general)

Worst one: FFVII

But I'm 100% sure that I would have loved FFVII much more if I would have played it around the time it came out.

Anybody counting the votes?

From reading the thread it seems that FFVI has the most votes for Best FF Game and FFXIII has the most votes for Worst FF

Final Fantasy IX is my favorite for everything from the best music, to the best fight system in a final fantasy, to the best mini games, best characters, best everything!

Final Fantasy XII is my least favorite, for the exact reasons I said above, but reversed. 

My favorite is between VI and VII, really can't choose between them. I loved the musics in VI so much. But my least favorite is XII, this is the only final fantasy that i didn't start all over again. I have finished and replayed every FF from the start (even the one on NES). But i just cannot bring myself to start XII all over again. After FF X didn't really like the way the FF series is heading. FF X-2, FF XII and FF XIII are probably my least favorites.

best: VII, VI, X (in that order)

worst: XII

Best: FF XIII, had change people couldn't handle and is a great game.

Worst: FF I, but still a great game.

My favorites is FF VII and X- both have great story and good music.

less favorite- Final fantasy X-2- i kinda like it but the transsition between the sweet yuna to a pop star was bad. i got to mention ff3(remake) i didn't even finish it.

Favourite: FF XIII

I loved the story,  the characters (Fang FTW!!!), the gameplay and, well, everything else.

Worst (out of the ones I finished): FF XII

In my opinion it had the worst main character(s) in FF games, the story didn't pull me in either. And probably my most disliked thing about FF XII was the combat system.

Out of the ones I finished this would be my complete list:

XIII > X > VIII > VII = X-2 = CC > IX > I > XII