For me it would be a toss up between VIII and XII,i have so many beautiful memories of both and have spent literaly weeks immersed in their game worlds.
In VIII I felt a connection with squall,and selphie and irvine always made me laugh
with XII i felt Vaan was a great lead although my favourite characters were Fran and Balthier(i love Gideon's voice, i've even spent half an hour talking to imperial guards in skyrim because of him )
I didn't particularly like X or X-2 although i still played them,same goes for XIII while the game looks and sounds amazing and the characters are really good i feel it doesn't have the "Magic" that the others had.