For me the order goes 9, 7, 11, tactics, 10, 8, 13, 12
9 has a good storyline and great characters, also each character having their own specific traits was brilliant, however it did leave some characters being rather pointless (Amarant).
7 is a classic even though people don't enjoy it. It was the first final fantasy game I played and it is brilliant. Materia system was awesome and the villain was brilliant. Cloud as a main character though was a bit lame, but you had Cid, Red XIII and Vincent, what more could you ask for.
11 is a good MMORPG and I lost actual days due to playing it (Playtime is approximately 19 days). The only issue was that the levelling up system was really really long, and most people had not patience for it and played WoW instead (the fools).
Tactics is highly addictive, due to the amount of classes and sub-classes you could have, as well as having a very strategic battle system.
10 was a good crossover from PS1 to PS2. The sphere grid is a brilliant system if you could master it. Also, Blitzball is the best side-game in any final fantasy series. But I did the voice acting in it, it was terrible.
I really didn't enjoy 8. I thought the storyline had too many issues and the junction system isn't as great as everyone says. Why would you make a system where you could junction a spell to your health, but you couldn't use it because your health will decrease.
13's graphics are brilliant, but you don't buy a game because it has good graphics. The storyline is clever, but the idea of running on the same path through the game is rubbish. The past games were free-roam so why not this one and yes I know you can reach a free roam area, but why wait until near the end of the game.
12 is possibly one of the worst games I have ever played. The free roam system had issues, and the gambit system was appalling. This game made me lose faith in the franchise.
Anyway that's my order and I know people will disagree, but that is my opinion