The best for me is Final Fantasy VI. I could list a million reasons why, but people who have played the game already know, and those who haven't have already read tons of reasons why. There is a reason it ranks consistently as the top Final Fantasy and is hailed by many as the best RPG of all time. It's my favorite game of all time and it's just great.
My least favorite would probably have to be XII. The game didn't move me or suck me in at all and I found the story boring. I admittedly didn't finish it, but I put in a good 40 hours into before sticking it on my shelf to collect dust for the rest of it's days.
I also haven't played II, III, XI, or XIII Although I'm not sure if XI should even be included in this discussion, as it's a completely different type of game.
My ranking would probably be something like:
VI, IV, IX, V, VII, I, VIII, X, XII
