Apart from Aeris' death I think that the part where you are in cloud's mind in the lifestream where Tifa's mom has just died was really sad (btw I don't mean sad as in stupid, I mean sad as in tear-jerking/emotional). Especially when she and her friends are in Mt. Nibel and they run away, but Cloud still follows. The music really adds to it aswell.
P.S. I'm not asking for other peoples opinions on what part they found the saddest. I just want to know if anyone else felt the same about that particular scene.