I think FF1 WSC (and FF1 Origins on Normal) is the hardest version of FF1. The NES version isn't as user-friendly, but the WSC/PSX version still has some things that makes it harder. The bosses have double HP (Chaos with 4000 HP instead of 2000), and the enemies seem to be a lot more aggressive and smarter. In WSC, Mages and Sorcerers just love to ambush you in the Ice Cave and kill your party before you get a single turn (Fire3 + Bane until you're dead, or paralyzing your party and slowly killing everyone), and Lich II almost always starts the battle by casting Flare.

The DoS version should be the easiest. The Ice Cave has become easy there just for the reason that you can save everywhere. The whole reason the Cave is so difficult in other versions is that you have to survive a long time without saving, and you're never really safe from an enemy attack that might decimate your party.