The beam sword has been in the series since the original. This obvious Star Wars lightsaber parody has always been one of the best weapons. The Ultimate incarnation favors the Melee version than the big blue one from Brawl.
I always got a laugh when sword-fighters like Link and the Fire Emblem cast picked it up. Does that make sense? No.
I also love how the beam sword acts a little different depending on the fighter. It's got great power whether used traditionally or thrown at a foe.