Fallout 3 Unique Weapons - Vengeance
Unique Gatling Laser (Big Guns category).
It deals massive damage and its ammo isn't that hard to find (uses EC packs). The only downside is that it is difficult to repair since Gatling Lasers are very hard to find. Also, this weapon degrades rather quickly.
FALLOUT WIKI INFO:
http://fallout.wikia.com/wiki/Vengeance