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.
If you're looking for power, and a chance to make this game become
really easy, this is for you. Here are the locations of the most
powerful items that a character can be equipped with:
Atma Weapon: Found in the Gateway to the Esper World (in the World of
Balance). See article 4.4.8.
Offering Relic: Found in the Ancient Castle (in the World of Ruin). When
a character has the Offering equipped and is instructed to fight, he/she
will fight four times in the same battle turn to random enemies (like
Cyan's Quadra techniques.) As far as we've been able to tell, this is
the rarest relic in the game, since there are only two of them (the
other one is in General Leo's inventory and can't be unequipped.)
Gem Box Relic: Found in the Fanatics' Tower (in the World of Ruin). The
Gem Box will allow a character to cast two magic spells in one battle
turn. The only caveat: you can't summon Espers while the Gem Box is
Ragnarok Sword: Found in Narshe (in the World of Ruin). Locke must be in
your party to get this one - the weapon salesman in Narshe has it and
can turn it into a sword. Casts Flare randomly in battle, and consumes
MP for mortal blows like the Rune Edge. See article 4.4.17.
Illumina Sword: Only available at the Colosseum. To get it, you must
trade the Ragnarok Sword. The Illumina is a sword of mystic purity - it
has all the same abilities as the Ragnarok Sword, but it casts Pearl
randomly and raises a few stats a little more than the Ragnarok Sword
Air Anchor Tool: Found in the Fanatics' Tower (in the World of Ruin).
See article 4.4.11.
Chain Saw Tool: Found in Zozo (any time). See article 4.4.1.
Economizer Relic: Found by defeating Brachiosaurs (in the World of
Ruin). Some Brachiosaurs carry this relic around, which can make it so
that casting spells will only reduce a character's MP by one (as well as
Strago's lores). Brachiosaurs are only found in a forest north of the
Veldt, which is shaped like a Tyrannosaur head. Good luck, Brachiosaurs
are the hardest regular enemies in the game.