Here is my ranking from hardest to easiest:
1. FF13. Some people think this game is easy (which is strange to me), but some of the bosses in this game (particularly a few of the eidolons) were brutal. It was a pretty fun challenge tho. For all FF13's faults, it has awesome boss fights that require a good deal of strategy.
2. FF5. Getting through the dungeons is quite easy, but some of the bosses in this game are beasts. I love the amount of strategy that the jobs system provides tho. You can literally make a very difficult fight much easier by having an intelligent job set up.
3. FF12. Some of the bosses were very tough, but what makes this game challenging is the constant need to grind.
Those three are the only somewhat challenging FF games. The rest are quite easy.
4. FF6. This a very easy game most of the time and it has a lot of overpowered moves for you to spam, but there are a few ridiculously steep difficulty spikes. Floating Continent anyone? I also find Kefka's Tower and the Kefka himself to be a little tricky.
5. FF4. I played the easy type, which lived up to its title (people have told me that the hard type is one of the most difficult games in the series), but some parts retained just a little bit of that old school challenge.
6. FF8. I know that abusing the junction system can make this game a joke difficulty-wise, but with normal junctioning, the game is not the cakewalk that some people make it out to be, though it's definitely one of the easier FF games.
7. FF9. This game is only tricky during disc one. After that, the game gets really easy, especially with all the handy support abilities you can equip.
8. FF10. This game has its fair share of challenging boss fights, but your party members seem pretty overpowered IMO, especially the summons. Also, the final boss is a pushover.
9. FF10-2. If you ignore a few of the difficultly spikes in some of the sidequests, this game is an absolute cakewalk. What's kind of disappointing is that the lack of challenge means that you don't have to really take advantage of all your jobs. You can pretty much plow through the game with two warriors and a white mage.
10. FF7. During my latest playthrough, I think I think I only died three times and one of those deaths was an accident while another was at the hands of the final boss. Honestly, FF7 is probably one of the easiest JRPGs ever. Limit breaks are just so ridiculously overpowered.
I haven't played 1, 2, 3, 11, 13-2, or 14, so I don't have an opinion on those.
What does your ranking look like?