it can be anything like

A Rocket launcher that shoots flaming chainsaws that explode

I want a grenade launcher that shoots grenades that explodes and throwing stars shoot out of it along with naplam and as soon as they stick into something...they explode as well. Then I want a pet rocket husky from space station silicon valley

Im thinking a weapon that is similar to the Needler from Halo 3, but when in mid air, each needle splits into 100 more, and they all aim for the person's head as heat seeking missles(sp?).

i want the gun that killed john f. kennedy, magic bullets included. then if it exists (not to trusting of religion, especially christianity) i want the tip of the lance that was used to stab christ, then im gonna duct tape that to the rifle as sort of a bayonet and then....... fuck it this is dumb

I would want a pistol that could morph into a large laser type cannon...
ex. the pistol from the anime series Trigun

Superfly's body, seeing as how it IS the best weapon. =P

And yes, JRO, that gun from Trigun is awesome.

clouds.
buster sword.

or, another weapon i would like from my fantasy would be a glove that allows me to shoot bursts of gigantic flaming balls from the glove.

an electro-magnetic rail gun that fires AP tungsten rounds at a mile a second, id be the worlds greatest assassin :P

i would want a magnetic railgun that shot 100 pound depleted uranium shots or something along that line and a infinite supply of depleted uranium