I say weapon because it gets to complicated with all the super powers you can get. Turning invisible, walking through walls, teleporting. So for this I want to limit it to sci fi weapons that at least attempt to have a tech basis. So like not the masters device that turns people into dolls. However we don’t care if we can completely explain how it might be done now. Like lightsabers are fair game. As for me I would go with the star gate zat guns as initially put in. Both lethal and nonlethas and very portable. I want to put an honorable mention for tthe minority report sonic shotgun that seems to run on wind up energy so you should never theoretically run out of ammo. although not so sure as there was no indication you could power up more than one shot at a time.
EDITED: Its a little late at this point as the answers have went way off the rails but in case it gets more replies my intention was a weapon someone might carry around and could use as one person. not a weapons system or vehicle. Also no robots or things that essentially are their own entities. It should be a weapon used in a weapon like manner. aim and fire. also that quote about sufficiently advance technology looks like magic. if it looks like magic to us instead of technology then its to far afield. I hope this makes it more clear what I was looking for. Honestly I would have like to not use weapons as a basis but there are just to many things when you look at all of sci fi.
An emp that blankets the world, but instead of electromagnetic it’s empathy manipulation such that it either eliminates those lacking empathy or forces them to be empathetic.
Goodbye capitalism, pedophilia, corruption, etc. We’re facing what I believe is a global empathy crisis through the rise of the far right due to global neoliberal capitalism. To secure profits, which we’ve tied to survival, we’re un-learning empathy, which was an evolutionary advantage for us. Capitalism rewards narcissism, a lack of empathy, being willing to exploit others.
I would set that empathy pulse off in an instant. I’m tired of seeing people brutalized around the world because of greedy people in power.
I think such a weapon is achievable once we understand genetics enough. Create a gene modification that makes people a little more cooperative and more interested in everybody else’s welfare - say 20% more - and infuse it into enough of the world’s water supplies to give the mutation to a majority of humans. Wait a few years and I think their behavioral changes would turn our whole world around. No need to design a perfect social system and use politics to implement it. It would quickly evolve on its own. .
Isn’t this the lore of the Reavers in Firefly?
Similar idea, but the “pacification gas” wasn’t a genetic modificationn. And of course how something works in a sci-fi story doesn’t depend on how it worked in a different one.