Clearly it is the Ultima Sword.

Why yes, yes I
am a Final Fantasy VII fan. Why do you ask?
In all seriousness, though, I would have to say that the Roman
gladius and the later
spatha that largely replaced it are the world's most devastatingest swords. They helped found an empire that hugely influenced the (Western) world we live in. Although, I do also have a fondness for the Egyptian
khopesh. It's pretty spiffy.