I voted FFVI, but maybe you should play TA first.

TA is a pretty bad game when it's compared to FFT, and it'll leave you with a bad taste in your mouth if you play it last.

VI, on the other hand, is so fraking good that after you suffer TA, it'll be a pleasant relief to finish up with one of the best games in the series (Many call it THE best, but I still hold that FF1 NES deserves that title)