This is my personal preference best to worst
Don't hate but please post your list

#1 - 7
#2 - 10
#3 - 8
#4 - 13
#5 - 9
#6 - 4
#7 - 5
#8 - 6
#9 - 12
#10 - x-2

pretty in order of what i enjoyed the most

Hmmm, haven't played them all and it's kinda hard to decide, but I'll give it a shot:

#1. 7
#2. 10
#3. 9
#4. 6
#5. 4
#6. 5
#7. 8
#8. 2
#9. 1
#10. 10-2
#11. 12

1. Final Fantasy Tactics
2. Final Fantasy 8
3. Final Fantasy 9
4. Final Fantasy 5
5. Final Fantasy 6
6. Final Fantasy 7
7. Final Fantasy 10
8. Final Fantasy 12
9. Final Fantasy 1
10. Final Fantasy 10-2
11. Final Fantasy 13

Those are all the ones I've really played. If you counted FF11, it'd probably be either #3 or #4 on my FF list, but most people don't even consider that so I don't want to make fanboys angry.

1. FF6
2. FF9
3. FF7
4. FF8
5. FF10
6. FFT
7. FF5
8. FF12
9. FF4
10. FF1

From the ones i have played.

FFX
FF Crystal Chronicles
FF7
FF8(have not finished yet so this list isn't quite accurate i'm sure)
Dissidia
FF12
FF1(not completed)
FF2(not completed)
Crystal Chronicles: Crystal Bearers(was bad)
FF Tactics advanced(was worse)

i forgot about crisis core, that would be my #4

6
4
3
7
8
crisis core

Ah geez yeah, forgot it too :S

#1 - 7
#1 - 10
#1 - 8
#1 - 12
#1 - 9
#1 - 4
#1 - 5
#1 - 6
#9 - 13
#10 - x-2

already liking 8 a lot..pretty sure it's gonna be going above 7