FFIV and FFVI have always been my favorites, although FFIV wins (probably due to massive nostalgia, it was one of the first SNES games I ever played). I think FFVI had the potential to be the best, but the World of Ruin was so anti-climatic.

FFIV's story just keeps building until the end. After a couple of hours of play in the FFVI WoR, the story kind of just stops and you end up doing a series of sidequests to find your missing party members. Although I appreciated the non-linearity of the sidequests, the story suffered as a result.

FFIV will never be surpassed, even with its awkward dialogue...