My vote goes to the original FF1 for the NES. The newer versions are much easier. The ridiculously small amount of space that you had to carry Weapons and Armor forced you to budget everything you brought. The new equipment is usually too expensive when you first arrive in a new town, so you have to go fight to save up the GP. And then the battle system is rather wacky. If you cast "LIT 2" on two of the same enemy, it could do 178 damage to one and 23 damage to the other. Buying 99 HEAL potions requires extensive pushing of the A button since you can't buy in quantity. There is also no means of filling up your MP outside of an Inn, except by using a "House", but Houses are expensive and if you do have one, you'll probably never use it because you'll be "saving it for emergencies". This, of course, makes magic worthless in almost all of the fights, except boss fights. When fighting a boss, the right magic can make the fight stupidly easy. However, because you only have a couple of casts of your best spells through the whole game, you can only use good magic against the boss, making your mage a waste of space the whole way through the dungeon. And once you get to a high enough level to have an abundance of lower level spells, you'll be up against monsters who aren't fazed by them. Also, there are no save points. In fact, the only way you can save the game is with a Tent/Cabin/House or by staying at an Inn.
To some, it may seem as though the game's difficulty relies on luck. For example, you'd need luck to avoid running into 8 zombies in the Earth Cave. And luck would decide whether or not they stunned all four of your characters immediately and then slowly kill them off. However, if you're good enough, you can run into 8 zombies and deal with it. You can cast HARM or HRM2. You can have all four characters attempt to run. You can cast FIR2. So it's luck that gets you into situations, but preparedness and strategy can save you. Except from 9 pirates, especially the kind that hang out in Pravoka.