ok my history is: played+beaten: I,II,VII,VIII,X,X-2. I am now playing through and i'm almost finished with VI.

Of these games, i believe that it is a very close race between VI and VII (no surprises, of course)
to me, VII had a stronger storyline, cast of characters, and role playing aspect.
VI had stronger graphics, sound, and overall completeness.

Don't ask me about the music, they both are awesome.
But still, i liked VII better. I loved the deep plot. I actually enjoyed the fact that you have to think about what is going on after each session of playing.

but even saying that, i know a lot of people that only played ffVII and therefore hated the series because they hated VII.