FFV - Ultimate Weapons Guide
In Final Fantasy V the ultimate weapons you can get for your characters are called the 12 Sealed Weapons and they come from the Sealed Castle which is in he northern - central portion of World 3 aka the Merged World. Use my screen shot above if you're trying to find the castle with these weapons in them or my screen shot below for the map location.
Unlike most of the other Final Fantasy games, there isn't a unique side-quest for each of the Ultimate Weapons. At the end of the game there's dungeons you have to complete which reward you with tablets, one of them (the pyramid) is required to beat the game, the others are not.
All you need to do to unlock the Ultimate Weapons is take the tablet to the Sealed Castle, which we talked about above and use it at the podium in the dungeon which is shown in the screen shot below. After you do this you'll be able to pick any three of the Sealed Weapons that you want. That's seriously all there is to it.
Navigating the Sealed Castle dungeon is easy, the first time you're here you'll have to fight Exdeath's Soul as a random encounter - after you unlock any of the Sealed Weapons you will no longer fight Exdeath's Soul, instead you'll fight the Zombie Dragons which were a part of the Sealed Castle in World 2.
As aforementioned there are a total of 12 Ultimate Weapons in FF V. I'd highly recommend not depending on the in game Optimize feature because it won't take the weapon's special advantages into account, it only equips the weapon with the highest attack rating.
Below is a quick list for you going over the strengths of each weapon.
Final Fantasy V Ultimate Weapons
Assassin's Dagger - 81 Attack Power; 1 Speed and 25% chance of instant death (to opponent) when attacking
Apollo's Harp - 45 Attack Power; Auto Attacks are a spell with 75 base attack power. Deals 8x damage to Undead and Dragons
Excalibur - 110 Attack Power; Holy elemental and +5 STR
Fire Lash - 82 Attack Power; 90 Hit Rate +2 STR and Speed as well as a 33% chance of casting Firaga when attacking
Gaia's Bell - 35 Attack Power; 99 Hit Rate and has a 25% chance of casting Quake (also makes Earth spells do more damage)
Holy Lance - 109 Attack Power, Holy elemental and +3 STR
Magus Rod - 40 Attack Power; Increases the damage done by Fire, Ice and Lightning attacks
Masamune - 107 Attack Power; Increased crit chance, always first turn in battle and when used as an Item casts Haste.
Rune Axe - 71 Attack Power; 90 Hit Rate and raises Magic Power by 3. Also can consume some MP during battle to deal a Critical Hit
Sage Staff - 45 Attack Power; Casts Raise when used as an item and deals additional damage to Undead targets. Also increases power of Holy Attacks
Sasuke's Katana - 99 Attack Power; Increases Speed by 1 and blocks physical attacks 25% of the time
Yoichi's Bow - 101 Attack Power; 90 Hit Rate and raises STR/Speed by 3. Also has an increased Crit Chance