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.