Avatar Roku said:
In terms of sheer usefulness, the Brush Gun and Anti-Materiel Rifle. For sheer coolness, for me, the simple 9mm Pistol, maybe Maria. I just love the good old M1911.
Not to be an asshole, but the 9mm pistol is based off of the Browning High Power.
http://fallout.wikia.com/wiki/9mm_pistol_(Fallout:_New_Vegas)
This is the M1911 (.45 Auto pistol) found in the Honest Hearts DLC
http://fallout.wikia.com/wiki/.45_Auto_pistol
OT: The fully modified Light Machine gun. 200 rounds of Armor Piercing oodness can cut down an entire Legion Assassin Squad with about half of that in under 25 seconds.