Start at FF1 it's super short, and sets the template for all of the rest.

1-6 are like playing the Evoland games if you have played those. The mechanics change with each new game but the basic game sounds, and system mechanics stay the same.

7-10 are the best, and where the games changed to 3D from 2D.

The stand alone 10 games after 10 are really forgettable.

11 is basically just an MMO

12 is spit into 3 and feels more like Fantasy Star than Final Fantasy.

13 is great back to the old ways mostly.

14 is MMO again

15 is like a split MMO/Offline game that is always online.

Just my opinions. Hope that is helpful.