Unlike other Final Fantasy games there is only one all mighty powerful weapon and it is called Zodiac Spear but unlike other Final Fantasy games it is one very hard weapon to obtain.
So what is the problem? Well there are four Treasure Urns in the game that you must not open if you wish to obtain the mighty Zodiac Spear. As the treasure urns have no distinguishable features on them (they are exactly the same as normal ones) you will have a very hard time trying to find the right ones, but do not worry, I will tell you exactly where they are and when they first appear.
Please note that in the International Zodiac Job System you do not need to miss any of the following mentioned Treasure Urns.
1. The Treasure Urn in Lowton opposite Dalan's residence.
2. Inside the Royal Palace of Rabanastre Cellars (so when you first enter the place from the Garamsythe Waterway) at the south-east corner.
3. The middle Urn in The Confiscatory (where you get back your weapons) inside the Nalbina Dungeons.
4. In the Phon Coast at the Vaddu Strand where there is a collection of Urns (4x4 box of them). You can either ignore them as you either get 200 Gil, or a Knot of Rust (if you have the Diamond Armlet equipped you have a 10% chance of getting Dark Matter), if you want to get all the Urns but the one your not allowed to then take a look at the diagram below.
Now if you did not open the four Treasure Urns mentioned above then when you go to the Necrohol of Nabudis you will find in the Cloister of the Highborn a room full of Urns and one of them contains the Zodiac Spear. Since these Treasure Urns do not respawn after you have opened them once you will need to be careful if you want to get all the best treasures from this room (that is if you have the Diamond Armlet equpped).
It is easier to just find the Zodiac Spear first (first coloumn when you enter, third row down), and then to equip the Diamond Armlet for the remaining Urns.