FFVII Ultimate Weapon Guide

Cloud's ultimate weapon is the only weapon that requires fighting a boss to obtain. To get Ultima Weapon, you need to defeat the Ultimate Weapon -- an enemy that can be seen hovering above a crater on the Eastern Continent. You can begin fighting Ultimate Weapon immediately after the encounter with Diamond Weapon, but you MUST have Knights of the Round in order to complete it before the party parachutes into Midgar. (Note : It is not necessary to have Knights of the Round to defeat Ultimate Weapon, it only allows the player to defeat him before he has a chance to land over Midgar)
To start, take the Highwind and fly to the area around Junon on the eastern continent. When you locate Ultimate Weapon, bump into him with the highwind to initiate a battle. You will not be able to finish Ultimate Weapon in just one fight. Rather, it will be a series of fights, both aerial and on the ground. During these fights, you can steal several items from Ultimate Weapon, one per battle. On the aerial fights, you can steal a circlet, and on the ground battles, you can steal a reflect ring. (Stealing on aerial battles will require the character equipped with steal to have a long range materia as well). At the end of each battle, Ultimate Weapon will flee. When you're looking for him again, you will usually need to bump into him 4 times before he finally lands somewhere. After each fight, he always spawns over the crater where he originally spawned, so keep that in mind. Once you have fought him enough to get his HP below 20,000, he will finally park himself over Cosmo Canyon, for the final battle. He has 20,000 HP in this battle. (Even if you did a massive amount of damage to him in the prior battle, he will always start with 20,000 HP).
Ultimate Weapon has two devastating attacks that the player should be prepared for. One of which, is Ultima Beam, and the other is Shadow Flare. Shadow Flare will always be cast upon the death of Ultimate Weapon, so be prepared for that. He will always cast it on the player that dealt the killing blow, so keep that in mind if you want the enemy skill. After he is defeated, he will drop Cloud's ultimate weapon - Ultima Weapon.
Ultima Weapon has 8 materia slots, with no growth. It's damage output is proportional to Cloud's current HP. (I.e. if Cloud's current HP is 9,999, Ultima Weapon will deal 9,999 damage... however, if his HP is at 3,278, UItima Weapon will deal 3,278 damage, or thereabouts.)

Barret's Ultimate Weapon is very likely the easiest ultimate weapon in the game to get. You can only get this weapon during the raid on Midgar, and Barret MUST be in your party for this to work.
Right after the fight with the Proud Clod, go up the staircase that leads to Hojo. Missing Score is on the second level of the staircase.
Missing Score is just like every other ultimate weapon, in that it has 8 materia slots, with no materia growth. The damage that Missing Score outputs is directly proportional to the combined amount of the AP of all materia in the slots of that weapon. It has an Attack power of 98, and an Attack % of 108.

Tifa's ultimate weapon can be found on Disc 2, in Midgar. However, in order to get back into Midgar, you will need to find the key to sector 6, which according to a man standing outside the gate, is located at Bone Village - the area where you found the Lunar Harp.
In order to obtain the key, return to Bone Village. Tell the excavator that you're looking for "Normal Treasure", and place all the units around the top of the excavation site. Then follow the same procedure as you did with the Lunar Harp. You should dig up the key to sector 6, and, if not, try again. Once you have the key, return to Midgar and enter. Head to Wall Market, and go to the tent that says "ITEM" in big letters across the top. Talk to the computer, and you will recieve Premium Heart.
Premium Heart has 8 materia slots, and no growth. Premium Heart deals more damage as Tifa's limit gauge fills. It has an attack power of 99, and an attack % of 112.

Aeris' ultimate weapon is obtained during the party's journey through the Temple of the Ancients. Once you find yourself in the room with the clock hands on the ground, set the time so that you can walk to IV (4). Princess Guard will be in a chest at the end of the tunnel.
Princess Guard has an attack power of 52, and an attack % of 115. It powers up whenever one member of the party is knocked out, and becomes extremely powerful when 2 are knocked out. It has 6 materia slots and is the only ultimate weapon with materia growth.

Limited Moon may be obtained after the Weapons attack Midgar near the end of Part 2. After they attack, return to Cosmo Canyon with Red XIII in your party, and enter Bugenhagen's study. A sequence will play out, ending in the party recieving Red XIII's ultimate weapon, Limited Moon.
The effect of Limited Moon deals damage based on Red XIII's current MP. It is exactly like Ultima Weapon, but instead of depending on HP, it depends on MP. It has an attack power of 93, and an Attack % of 114.

Cait Sith's ultimate weapon can only be found during the raid on Midgar. You must return to the Shinra Headquarters in Midgar, and go to the athletic room on the 64th floor. Go to the locker room in the back, and open the locker on the top left corner of the small room. HP Shout will be in that locker.
HP Shout is identical to Cloud's Ultima Weapon in its effect - it deals damage based directly off of the character's current HP, so HP plus materia is a good boost here. It has an attack power of 95 and an attack % of 110.

The old man that you get Yoshiyuki from earlier in the game will give you this weapon when you talk to him near the end of disc 2 and into disc 3. Talk to the old man in between the item shop and Cid's house who is staring up at the rocket. Talk to him 3 times, and he will give you the Venus Gospel.
The Venus Gospel deals damage based on how much MP the player currently has, just like Red XIII's. It is recommended to get MP plus materia to maximize damage. It has an attack power of 97 and an attack % of 103.

Yuffie's ultimate weapon can be obtained any time after capturing the Shinra sub. Drive around under the water until you find a sunken plane, the Gelnika. If you're doing this early, be prepared for very powerful enemies in this area. Once you enter, go into the room above the save point, and collect all the items in there. Conformer will be among those items.
The Conformer is the ideal weapon for the Morph Materia, since there is no damage penalty when using morph with it. It has an attack power of 96 and an attack % of 112.

Vincent's ultimate weapon may be found near the end of disc 2, and during disc 3. Take either the submarine to the waterfall in the center area of the western continent, or use a mountain chocobo. Take your party inside the waterfall, with vincent in your party, and a sequence will play between Vincent and a woman named Lucrecia. After the sequence finishes, your party will immediately be evicted from the waterfall. As soon as this happens, fight atleast ten random battles, then go back inside, and Vincent's ultimate weapon, and level 4 limit break will be on the altar.
Death Penalty's damage increases as monsters are killed in battles. It has an attack power of 99, and an attack % of 115.