The Soul Edge is pretty intense for a sword - it was born when a normal sword was bathed "in blood and hatred" (without Mr. Bubble, no less). In the 2nd game, the sword would mutate its form to fit whomever was using it at the time (for Kilik, it was a staff; for Ivy, it's a boob sword; etc.) and - despite being a weapon - is the primary antagonist for the entire series. Wielding the sword even causes you to lose health - and, as legend has it, will curse anyone who uses it. Maybe the lesson here is to avoid swords that have their own eyeballs.
Okay - technically this should count for two entries, but the Blades of Chaos are a package deal. I mean - flaming swords that can be whipped around thanks to the chains attached to your arms? Is it even possible for any sword to be more badass than that? Maybe if they also shot tinier swords out of them and caused your enemies eyes to explode, but that might be getting a little unrealistic. And while Kratos loses the Blades and they're replaced by the Blades of Athena (and then the Blades of Exile), these are all pretty much variation on the original Chaos version. And there is scarcely a better tool for Kratos' brand of ultra-vengeance than double-chain-super-swords.