This is a tough question considering how many Weapons exist within the Final Fantasy series. I would consider Shinryu and Omega from FFV as Weapons as well, however. In my opinion, Ultima Weapon from FFVIII and Ruby Weapon from FFVII were the "coolest" of the Weapons. As for difficulty, none of them were hard when one has maxed-out stats, but at lower levels, Ultima Weapon from FFVIII was insanely difficult, especially without the Auras and Holy Wars.