FF1's Party Creation - Yet basic, had the Jobs I neeed
FF2's Key Term System - Found it frustrating at times
FF3's Support Character System - Never played
FF4's Introduction of ATB, and everything - Enjoyed it
FF5's Job System - Fantastic, though as you levelled up..I hated waiting to learn the last ability of a certain job
FF6's Esper System - Utterly e. The only thing Espers were good for, were for learning their skills. Summoning them, that was the utter e bit, nead to clarify that.
FF7's Materia System - Boring. Plain. Hated how your HP went down with everything you equipped.
FF8's Junction System - One of the best systems I've used - yes, Draw may have been a pain, but once you mastered Junctioning, you didn't have to waste your time always summoning.
FF9's ATE System - Can someone explain to me what ATE means ; - If you meant how the accessories gave people abilities to learn, then that's ok.
FF10's Sphere Grid System - Hated it. I did not appreciate having to waste spheres going through things I didn't like.
FF10-2's Mission System - Easy to use, but it made battles too easy. xD
FF11's Online System - Not played
FF12's Gambit System - Haven't used as such (obviously as I have not played it)
Other - Could I include the Clan sytem from TA in this? o.o

I can't really vote for 1..because the ones I do like, I like them equally.