As a complete package & a truly seminal title VII is the greatest & most fondly remembered of the lot. It has the best atmosphere, characterisation & plot of any game in the series, in my & many other's opinion. Plus it has the greatest villain in any game ever. This is a fact.
Plot-wise I'd say VI comes in second, but should be considered in light of the fact that it's a SNES game, with all of the 2D stylings & sometimes clunky interface that that entails.
For me it's difficult to choose between VIII & IX as I love them both.
VIII is probably the most measured of the series, features a cast of precociously moody teenagers & has its main focus as a love story between the two main characters. Sometimes this is excruciatingly heavy-handed & juvenile, but at others it really works & stands apart from the other games in the series in this regard. Its magic system is the most complex of any Final Fantasy & is a love-it or hate-it affair, much like the whole game really.
IX is quite bright & breezy, something of a return to the series' roots, & for the most part is a joy to play. But it loses its way a little in the final 3rd & the simplicity of its game systems are either a bonus or a failing depending on your perspective.
Final Fantasy Tactics is superb, with more depth & opportunity for experimentation than any other, but it's very much an acquired taste.
In my opinion the ones to avoid are: X & XII.
X looks nice, but in all other regards is piss-poor. The plot is shite, the characters are a bunch of dullards & the sense of making your way across a vast, foreign land is non-existent, due to the absence of a traversable world map. In my experience those who tend to rate it so highly are those for whom it was their first Final Fantasy. Speak to anyone who played the SNES or PS1 games first & they'll invariably rate those earlier titles higher.
XII was one of the biggest gaming disappointments I've ever had. It looks lovely, marks a real change for the series by making combat real-time & won game of the year awards all over the place. But the plot is frankly dreadful, the characters are completely forgettable & the feeling of endless level-grinding never goes away. I'm in a minority with this opinion though, as millions seem to love it.