FFII and FFVIII had really steep learning curves, but once you learn the system, you can abuse it to no end (can you say OHK Astaroth with no blood sword?), so I can't say either are THAT difficult. FFIV has a lot of bosses that can't be beaten strategically unless you just level grind. FFI is interesting in that poison is actually a major problem. I can't think of any other game in the FF series where I panic and run away whenever I encounter an enemy that can poison you. It also has WarMECH. I guess FFI or FFIV (the normal versions, not the GBA versions) would be the hardest that I can think of offhand.