In the original FFI, the Masamune was the most powerful weapon, and the Excalibur was the second most powerful. However, a variety of weapons from other titles in the series were added to the DoS version, some of which exceed Masamune in power. Masamune is near the end of the game, on the same level of the Chaos Shrine where you fight Tiamat for the second time. There is no 'Atma sword', although that is a clear reference to the Ultima Weapon. In FFVI, there was a sword called the Atma Weapon that powered up based solely on HP. Atma, however, was a mistranslation, corrected here. The Ultima Weapon can be found at the end of Whisperwind Cove.