In Final Fantasy VII, the Apocalypse sword is a weapon for Cloud. It has three single materia slots which have triple growth (a must-have for materia like Knights of the Round). Attacks with this sword are successful 110% of the time, with an attack power of 88 (max of 100, logarithmic effect scale).
The Apocalypse sword is significant because, unlike all but one other weapon in the game (a two-slotted 'scimitar' for Cid Highwind, thanks to Eidolos for the info), it has triple growth. And unlike every other weapon its growth total exceeds 8x, i.e., 3 slots with triple growth is 9x AP. The next best for total AP earning is actually a four-slotted double growth weapon, or an eight-slotted single growth weapon.
Where to find it
The Apocalypse sword waits at the very end of Frog Forest. Frog Forest is a special area, containing fairly simple puzzles, in the mountains southeast of Cosmo Canyon. To get there, you either must have a green Chocobo or have defeated the Ultima weapon.