Either 7 or 10. They're both pretty similar, except 7 doesn't have an attack function, and 10 has way less limits for each character and they're also easier to gain. I like how 7 had lots of limits and also a 7th hidden one.

9 is annoying when you get into trance against a weak monster.

6 is annoying because you almost never get to see them

8 had cool limits, but it was too abuseable like someone mentioned. Quistis' blue magic was cool.

7 it is.