I don't think any of them really qualify as hard, but FF5 is the hardest of the ones I've played (1, 4-8, 10, T). FF1 was really hard... when I was 8. Now I can beat it with one blackbelt, if I can survive the first couple levels. FFT had the potential to be hard, but the system was way too abusable and the computer just couldn't keep up past chapter 1 or 2. Try Tactics Ogre: Let Us Cling Together if you want a harder tactics game, many of the people who worked on it went on to work on FFT after.