I’m partial to the plastic handgun crafted by John Malkovich in In the Line of Fire.

A sleek .22 caliber gun, custom made by a man named Lazar, for one of the greatest assassins in the world, Francisco Scaramanga. The practical implications of this weapon are astounding. Its incredibly easy to get this gun past customs, due to the fact that the weapon is collapsible and can be disguised as a variety of common items. This means that, even if you were searched, they would be none the wiser if they were to stumble upon the pieces. The main failing of the gun is that it carries only one round. The bullets are also custom made, and thus extremely rare, meaning that it is necessary to plan every shot and never waste it. This gun was designed for one man, because only that man has the skill to use it. Only one other man has the skill to use this gun, and that man is the target.