Definitely Final Fantasy VII (1997) because of the memorable characters and their relationships and the in-depth story. The story is the major part of the game for me as it has all kinds of twists and plot-holes which makes it all the more interesting to find out once you complete it, or discover from an expanded source in another game (the FFVII Compilation). My favourite characters are: Zack Fair (not in the original that much) Cloud Strife (main character) and Sephiroth (main antagonist).
The music is also a fantastic addition, arguably Nobua Uematsu's best work. Aerith's Theme and the Final Fantasy Theme are perfect examples.
This, the majority of the compilation, FFX, FFX-2 and FFXIII are the only ones I have played.
I think Final Fantasy XII was the best Final Fantasy game I have ever played because of its unique battle system and challenging quests.
i will also pick part 6 runners up in order are 4,7,8,10
Final Fantasy VI. Tons of great characters in an epic, dramatic story with beautiful music. The fact that it has a decent translation is what puts it over VII for me, which had kind of a weird translation quality. It's arguably the most ambitious console game released up to its time and you'll have a hard time not getting attached. It's also got a good, easy-to-get into gameplay style.
Also: has the best villain ever.
V and VII are my runner-ups.