6. Soul Edge (SoulCalibur series)
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.
5. The Blades of Chaos (God of War series)
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.
4. Energy Sword (Halo series)
The Energy Sword turned Halo's multi-player into a Slappers Only nightmare-scenario: suddenly close-range combat was a terrifying race to grab the sword. Otherwise, someone could silently sneak up behind you and lunge towards you with the Energy Sword in hand and you would be helpless to stop them. The distinctive sound made when busting out the sword was enough to make anyone in the area instantly lose all hope and make their peace with God. And then, just like teenage sleepover, getting teabagged was all but certain. That happened to everyone in high school, right?