The first three games have the hardest dungeons, because unlike the newer ones, you can't save inside of them, and you often have to go through many levels, and you are probably low on items. Not being prepared makes the battles ten times harder, but you don't really die a whole lot in them. It's just insane high damage, but you have the ever useful insane high cure.

FFIV has mini-bosses, but after you get the ultimate weapons, Zeromus is marginally easier. Save points are in some inconvenient areas. V is just wierd, but then certain bosses rip you apart. VI is just the underleveled-ness of certain parties.

FFVII is a joke. You could even place the save point in the WORST possible area, and still get out unscratched. I haven't played VIII, yet.
FFIX is difficult and wierd, because the battles are both annoying and tiring, considering you have to compensate for the way the paths curve and twist and contort.

X is easy (healing save points == nice way to level), and X-2 is... well, I have to make up a new word for it. In essence, stop playing video games for the rest of your life if you actually take damage during a normal encounter.