It's obvious a lot of people are gonna choose lightsaber and it is an epic choice but think about this there are a lot of really cool sci fi weapons LASERS frikin LASERS come on bros you can't tell me a laser would be awesome and I know a lightsaber can reflect lasers but think about this a Jedi cannot go up against a laser mini gun beat that Luke!
Lazer canon: the Best disintegration scene i Think is from the film the lost empirer from 1985. Ind the last scene the actor Linda shayne get blasted to dust by a Big lazer canon.
Movie based would probably gonna go with ghostbusters proton pack... Cross the streams potentially end life in the universe. Not much better than that.
Non movie based I'd probably go with the WH40k Vortex Blade the Soul Spear.... A weaponized black hole.
Weapon? Definately between a Xeelee Starbreaker and a Time Lord De-mat Gun/The Moment.
About the Starbreaker, it is difficult not to like a star busting handgun that can force a Supernova in a star that is not capable of going Supernova, and has yottatons of firepower in its lowest setting.
The De-mat Gun, well it erases whatever I shoot from existence. The Moment does the same to entire galaxies.
The Wave Motion Gun from Star Blazers...one blast from that and anything in it's way is suddenly gone. It's not portable, I know, but if it was, what a wallop. The only down side is the charging time and the cool down period before recharge.
Judging by the post's and the obsession with the lightsaber I think 90% of the posters here probably wernt even born when "Krull" was out. The glaive was a large portable golden star that had extracting blades that came out of it. This weapon could be controlled by the users mind. therefore it was a ranged weapon. The beast in which it killed was like a 40 foot alien that shot huge energy beams from its mouth and they could'nt event dent the glaive.
Lightsaber may be cool but no match for the glaive.