Title explains all - can be fantasy, sci-fi, or some gun-type thing that could exist, but doesn’t.
Mine are: The Alchemist - The Neutronium Alchemist - Wikipedia
Best doomsday weapon ever - basically generates ultrastrong gravity. On strong setting, can create a black hole. Drop it into a star - bye-bye star!
On the weak setting, it rips neutrons from atoms, gradually causing a build-up of neutronium on the outer surface. It dropped into something dense (like a gas giant or star) the build-up because significant, and exponential. The result - something not quite a black hole that makes the star go nova - bye-bye solar system!
Coherence gun - Miracle of Science - Wikipedia
Kick ass concept. Kick ass name. Not a decoherence gun, those things are lame. The coherence gun collapses the wavefunction of electrons in atoms, (making them coherent,) which sends the electrons crashing into the nucleus. Woot!
This, too, would generate neutronium. Meh, some people like leather.
The light saber of course. I think of it as being not as clumsy or random as a blaster, but an elegant weapon for a more civilized time.
On a “Physics of Star Trek” documentary (can’t remember the name but I believe it was on A&E), it was theorized that the light saber would have to be plasma. That’s why I was never able to get one no matter how many magnifying lenses I used over the flashlight.
Soul Edge. I’ve always been fond of mind-controlling weapons, and Soul Edge is both awesome looking, and has the added bonus of changing into whatever kind of weapon the wielder is used to using.
Soul Calibur also has that last bit of awesomeness, and it looks nice, too.
The Soft Weapon, from one of the stories in Larry Niven’s Known Space. It’s basically a very deadly Swiss army knife: On various settings, it’s a high-power computer, or a jetpack, or a variable sword, or a long-range cutting laser, or a flamethrower, or a disintegrator, or a ranged total conversion beam.
Also from the same series, there’s the Wunderland Treatymaker. It’s a modified disintegrator that, if both beams are used at once, will cause a current to flow. It’s also the reason an entire planet got re-named “Canyon”.
I’m fond of the PPG (Phased Plasma Gun) from Babylon 5. It’s a cute little thing, looks about like a holdout pistol, makes a distinctive mechanical whine sound when armed (like a camera flash, but manlier), and fires plasma bolts that can burn through doors and sections of walls if necessary. Also, the plasma bolt does a cool light distortion affect due to the heat.
The Marauder power armor from Starship Troopers. From what I understand, this was so cool, it spawned an entire genre of Japanese animation. I have yet to watch the new Starship Troopers movie to see if the Marauders there are nearly as cool (probably not, since flamethrowers, grenade launchers, nuclear bazooka, jet pack, and the Tweeter would probably altogether be a little over the top, even for that movie )
Reason wins on the name alone. “Oh, I’m sure he’ll listen to Reason.”
As an honorable mention, the Heron Mark Sword from Wheel of Time. This one is actually not my favorite because of how it was depicted in the books, but because of how the licensed replica actually looks. It’s a beautiful sword.
Green Lantern ring is very useful, but I have a long term liking for Katana’s (of Batman and the Outsiders fame) sword, which absorbs the souls of its victims and makes them available for supernatural advice.
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