Easiest: Probably FFVIII because the Junctions made you practically invincible. FFVI also had things that made it very easy, such as Osmose and the Moogle Charm... and the fact that you keep all the EXP you've gained even if you die. This means you can always walk through the dungeon again and again, and even if you die, you'll still get stronger, and eventually you'll beat the dungeon.

Hardest: I'd say FFIV Hard Type because it was very tough, especially with the max speed. Even the normal monsters took me a long time to beat in the end, the final dungeon is really long and the final boss is probably the hardest in the series. FFT and FFXII are somewhat hard (I still haven't beat FFXII, though), but I still think FFIV Hard Type is the hardest.