Nothing was ever hard. But the ones I will rank the most difficult experiences I've had:

Rapps when you're low level, disc 1

Materia Keeper because I just got Vincent from his coffin, and used his Galian Beast which does Fire-elemental damage....Materia Keeper absorbs Fire and you can't control Vincent at this point. So I had to kill him off with my other players and in the process got rocked hard by Trine and the Hell Dance.

Carry Armor got the best of me once. Never again did I let his antics trick me again. And now I find it repulsive to have a game over in FF7 unless you lose in that chocobo race in the Corel prison while NOT using that R1+L1 BS.