While I enjoy and appreciate most of the series for their own reasons,
FFVI is the best.
When FFVI released for the first time it was a revelation. As a kid I didnt think games could get any better.
The colors, the music, pacing, the intro(!) - the shear amount of characters, and they were all cool. The gameplay - even implementing Street Fighter motions for some.
FFVI even had 2 player co-op which we used all the time back then - enjoying the story with friends.
Years later FF7 launched and the world went crazy - even non-gamers - but after playing it I didnt get the hype. It was all flash and FMVs. There was a significantly less characters to play as coming from VI. The gameplay itself was leagues below FFVI. While FF7 was still cool and had its good bits, it was ultimately shallow eyecandy in comparison to FFVI.
Years later, FFXII is the only other entry that came close to FFVI. A perfectly realized world - combined with a hardcore slant to the gameplay, where status effects had to finally be taken seriously. A more serious political style story and characters is exactly what the series needed to finally grow up. With so many similarities in look style and enemies - FFXII is a psuedo sequel to Vagrant Story in my eyes. A shame Matsuno didnt quite finish his masterpiece - but its still a great game.