My favorite sword out of the ENTIRE Final Fantasy series I think would have to be the Lightbringer - "Legendary Sword of the Heavens" from FFVI Advance
I believe it was called Illumina on the SNES release. Let's take a second to see why it's my favorite sword! Here are the sword stats

Strength......+7
Speed.........+7
Stamina......+7
Magic.........+7
Attack........ 255
Evasion......+50
Magic Evasion...+50
Plus the sword has a 25% chance of casting Holy!

A sword amongst the gods if I say so myself!