Final Fantasy I and II are probably two the the hardest Final Fantasy games, if only because of ancient gameplay mechanics and frequent random encounters. Otherwise, if you know how to play through them, you shouldn't have much trouble.
The Dawn of Souls version of the games, although enjoyable, was extremely toned down in the difficulty department, possibly to cater to the newer generation of players who rightly shouldn't have to put up with aforementioned ancient gameplay mechanics. FFII wasn't as toned down as FFI, however, but both games were nerfed quite a bit. Examples include the ability to save anywhere (both games), the ability to hold 99 of every item (both games), the max level becoming 99 (FFI), more proficiency points gained after using a weapon or spell (FFII), and so on.
However, the Dawn of Souls version also has two new bonus dungeons (or a bonus campaign in the case of FFII), and many heartwarming references to the later games in the series, and many of the bugs that plagued the older versions of the game were fixed.
Anyway. I own the NES version of FFI, as well as FFI and FFII in Final Fantasy Origins. I believe the PlayStation version is the best incarnation of these two games, they both retain the unnerving difficulty as well as having beautiful graphics and near-orchestral quality music. The Bestiary, Item, and Art Collection (Amano <3) are great additions as well. I do enjoy the extra content in Dawn of Souls, but it'd be nice if they didn't completely destroy any difficulty these two games may've had, even if the intention was nice.
