I go for the Onion Sword. Not only is it stronger than any other sword in FFIII, there is more than one of it. Imagine a sword stronger than the Ragnarok in each hand...
It is also the hardest weapon to get, besides ExcaliburII in FFIX.

Masamune and Murasame are entirely different weapons, for those who are still debating that. People should read more legends...