If you didn't like III for it's difficulty, then I can't recommend I or II (except the GBA versions, which reduces the difficulty considerably)

IV requires a small amount of strategy since, from the beginning to the end, the game picks your party (again, except GBA)

V has a really nice strategy element to it, which involves taking certain abilities from one job and crossing them with other jobs. Other than that, it' just a longer version of III, but is much better in every way. The main villain has a smurfing pimp cane.

VI has great character development, and probably one of the best senses of strategy in the series, but it's also one of the longest, with all the side-quests in the second half (most of which you have to do if you don't want to suck) are fairly time-consuming.

VII has a killer story, and a neat battle system.

VIII has boring characters and a stupid story, topped off with the WORST magic system in the series. Don't play this game unless you have a high tolerance for pain.

IX is my favorite, as it has the best characters of the series, and easily the best plot. While the strategy is a little lacking, it's definately not as bad as IV (which is still a great game)

My recommendation would be (based off your preferences) V VII or IX