Alien device from South Park, basically because we played with cheats I would spam it and piss off my sister who would be stuck their singing like some ass =P

Anyway a more traditional answer, for aesthetics it goes to the Brotherhood in FFX

Notable mentions to 90% of the weapons in the Musou games =D

Energy Sword from Halo
Big Daddy Drill from Bioshock (or Skyhook from Infinite)
Sorrow/Mantis doll from MGS4
Portal Gun
Auron's Katana

Blades of Chaos. Stylish and a great combination of power, range and combos.

Man that's a tough one, probably the flak cannon from unreal tournament. Also I don't remember the name but I believe there was a gun in turok 2 that drilled the enemies brains right out of their skulls.

Tira's ring blade. Its just awesome.

Mmmm, I really can't decide among these:

Chicago Typewriter (Featured in various games, but the Resident Evil 4 version of it)

I've never been a gun enthusiast, but there's something about the design itself of this gun (It is known by Thomsom M1 if I am not mistaken) that I've always particulary like. Maybe it's the blocky aspect of it (mind you, I prefer it much more with the rectangular magazine, rather than the mafia-style round one); also bear in mind that this weapon not only destroy bosses with a few shots, but also provides you with infinite ammo.

Metal Gear Solid 3's EZ gun.

This is the ultimate espionage weapon. Not only makes you almost invisible just by having it unholstered in your hand, no matter how flamboyant you are, but it is also a perfect weapon for those who, like me, prefer to preserve life rather than reaping it. Also, if someone is bothering you, just EZ'd his face, then put his body into a locker.

Oh, also, infinite ammo. Yeah, because.

Metal Gear Solid 2's High Frequency Blade

I love katanas, and this particular sword not only gives you the ability of deflecting (some) bullets, but also, you can switch to a non-lethal blade mode on which no matter how many times you strike an enemy with it, you won't kill him, but rather, knock him out. Another perfect weapon for badass pacifism crowd-control.

The Evil Within's Agony Crossbow:

Not a fan of crossbows and their rise in popularity after The Walking Dead, but there's something about the twisted design of this weapon that speaks to my sadistic nature. The fact that you can throw different types of arrows and the crossbow itself adapts its structure to accomodate your arrow of choice is awesome.

Half-Life 2's Gravity Gun:

Playing around with this's gotta be one of life's biggest pleasures.

Fallout 3's special Fat Man: Experimental MIRV

Just for when you feel like going out with a blast.

Same.

Tentacle > dildo (Only talking saint row ofc)