The ultimate weapon is the Onion Sword, although it may only be used by the Onion knight
job

The onion sword: You may achieve this by going to the near top of crystal tower [crystal
tower is not very reccomended unless you're a very high level] Another way to get it is by
going through the secret dungeon [not really a dungeon. But that is even less reccomended.
It has the secret boss, Iron Giant, which is even harder than the last boss. You must
defeat the Iron Giant to be able to get to the dragons. Yellow drops swords/helm or
something else red helm, gauntlet/sword/or shield green sword/shield/gauntlet helm, armor.
Each dragon gives a different piece of the onion knight gear, All give swords and shields.
Also not reccomended unless at high level.

To get the job onion knight and access the dungeon: To get onion knight, you must send 7
wi-fi messages to other pc/players of FF 3. Then you must send 4 messages to Topapa. After
you do that, and get the 4th and final message, go to ur,[located in floating conitnet,
near starting cave] and talkto Topapa. After, he will ask you to save the three children.
Once you get the mission, you must go to the starting cave, where FF 3 began. Then, when
you get deeper in the cave, three children need you r help. [three bombs will be there. I
reccomend being level 50, although this is fairly easy.] After you defeat the monsters,
the children will give Arc a crystal fragment that will unlock onion knight job.

To access dungeon: Again, you msut sedn 7 wi-fi message to another player. Then, you must
first send 4 mesages to the 4 old men first using mog net. [located in the 2nd airship,
ask moogle to use it.] After sending 4 messages to them, send messages to King Alus, 4
messages also. [Now remeber, you may only send 1 message an hour, so, if you want to send
more than one, change the date/time by a day or hour by accesing the clock/ ds
settings]After, you will get a quest. Go get the fast airship [not the one with the
moogle.]and search near the floating continent where you will find a big hole. Dive in
there, and then you will see a patch of sea weed. Go in, search a little there, and then
you will enter the secret dungeon. [This is where the Iron Giant is. DO not fight him
unprepared!]