Well, Atma, Emerald, Ruby, Ultima (VIII), and Omega are all relatively easy. Omega from FFV was somewhat of a challenge, but that raises the question of whether or not it's actually a Weapon. I'll say that it is, simply so I can answer the question.

I'm surprised no one has tried to suggest one of the "false rumor" Weapons in FFVII, and I am profoundly thankful for that.