Part One: Weapon
This really should be a no-brainer. The best Sci-Fi weapon that comes to my mind is the phaser from Star Trek. Not the mini-vacuum cleaner looking models from TNG onward (though I would want the versatility of those models), but the classic one seen above. There is something comforting in its almost pistol-like qualities (though it also looks a little bit like a hot glue gun). More than that, it is the weapon Kirk used while he was kicking ass across the galaxy – at least when he wasn't using his hammy fists or constructing a bamboo cannon to shoot rocks at a rubber-skinned lizard.
It is also the weapon that can vaporize the cover your enemy wants to hide behind. Not that the Federation forces used it to its full advantage most of the time. Need to kill somebody? The phaser willcut through nearly everything to get them killed. It can vaporize a building or a boulder. Hell, you can use it super-heat a pile of rocks (in lieu of a campfire) without risking the rocks exploding on you. You can also set it to stun if you feel like it. I guess it is possible there would be a situation where killing the enemy wouldn't be necessary.
|The various mini-vac looking phasers (and one that looks like a garage door opener).|
But as much as I like the phaser – and my strong preference for Star Trek over Star Wars – the weapon I would take over any other would be the lightsaber. I mean, come on, its a fucking laser sword. Who doesn't want a laser sword? And one that, with the proper training (and a little luck) can be used to deflect all those little red bolts of light people might be wanting to shoot at you. Never mind the whole Jedi/Sith mythos – the lightsaber kicks ass because it demands the user to get up close and personal. It means the wielder means business. Plus it can apparently cut through anything (given enough time).
Is it as versatile as a phaser? Hell no! As a matter of fact, I'm pretty sure the hand phaser could vaporize Vader as easily as it would anyone else. But the hand phaser is kind of like a smart phone in that it is the technological tool that can be issued to everyone. The lightsaber is a badge of honor, or at least the mark of a dedicated warrior. Compare it to the ridiculous Klingon Bat'leth, the go-to melee weapon of the Star Trek universe. Are you done laughing? How about now?
The lightsaber is one of those things that I think so many people would love to be real that if one were to see one if use right before their eyes the reaction wouldn't be that can't be real because lightsabers are from the movies but rather Holy shit, that guy has a lightsaber!. No one is going to have that reaction to a phaser. We'd sooner expect William Shatner to win an Oscar than to see somebody shooting beams of phased whatever at anything in the real world. But there is a real hope that somebody would make the lightsaber. Almost as much as we would all love to have the Predator's personal cloaking device.