You know, the strength of the Ultimate Weapons is dependant on different aspects; each weapon's got its own advantages and drawbacks.
What weapon do you think got the best mechanics?
Ultima Weapon / HP Shout:
Totally wonderful with high HP, but will suck once your HP gets low
Missing Score:
Not that great by itself, but rules to heck with lots of AP equipped
Premium Heart:
Depending on how much the Limit Bar is filled, this weapon = suckiness or ruleness
Princess Guard:
If your party members are in danger, this weapon will power up lots; it's only got 7 Slots, but you can actually level up Materia with this weapon
Limited Moon / Venus Gospel:
High MP will guarantee great, wonderful power; low MP will put Red XIII and Cid to shame
Conformer:
Powers up according to the monsters' Level: the stronger the monster, the stronger Conformer becomes
Death Penalty:
You'll need to put a damn lot of effort into this one, since it powers up according to the amounts of enemies killed; but once finished, it'll stay at maximum strength, without being dependant on anything at all