This article is from the Final-Fantasy and other Square Soft Games FAQ, by firstname.lastname@example.org (Nick Zitzmann) with numerous contributions by others.
The King Sword, actually, is the game's best weapon: It can be used an
unlimited amount of times, and can critically hit _any_ monster. Too bad
you lose it at the beginning of the game...
Aside from the King Sword, the Glass Sword is the best weapon in the
game. Only the Nuke Bomb can do more damage than it can, and that can
only be used once. The Glass Sword can be used 50 times!
You can find it in the upper levels of the Tower. While proceeding up
the Tower, take a look in the room to the right of the stairs on each
floor up. The Glass Sword is in one of these rooms. The Aegis (Aezis)
Shield and some other goodies can be found the same way.