Final Fantasy Tactics, without a doubt, is the hardest FF.

And the FFIV release of the Playstation IS the original (hard-type) version. Just because they didn't translate it correctly doesn't change the system.

And FYI, the easy-type version was released after the badly numbered NA release on the SNES. The SNES version was the original, with was made easier for us dumb non-Japanese people. I've played a rom of the easy-type version, and it was pretty different of the SNES version here.