VIII and IX. VIII's battle system is among the best in the genre, if you like your battle systems to have a little complexity like I do. These two also have the best soundtracks in the series, by far, and great stories in my opinion. And IX is a very beautiful game.
VI is alright, but the battle system is (for the most part) rather boring, and the characters tend to be way too.. exaggerated. It's as if the writers were trying way too hard to give them unique personalities, and many of the characters just come off as one-dimensional cliches. There's the mad clown, the skirt-chasing prince, etc., and any time you see one of these characters, they will do their best to make damn sure you don't forget whichever mold they fit into.
IV has an excellent story and a writing style which is unique to the series, but the battle system is a bit bland. V has the best battle system of the SNES era, rich in character customization/development, but the rest of the game (story, soundtrack, ..) are largely unmemorable.