Which of the weapons in Fallout: NV do you feel you can draw the most overall use out of, as in a weapon that you can trust as your main armament for a large portion of the game?
My #1 pick would be the Laser Rifle, I picked it up very early in the game at lvl 6
The power fist can also be obtained fairly early and stay useful throughout a long game, but as a melee weapon it is more situational since you have to get to your opponents first. The 12.7mm submachinegun, although introduced a bit later, was another weapon that I drew a lot of utility from with one of my characters.
But can anything rival the Laser Rifle for sheer utility value? Sure it breaks down quickly, but repair is cheap and after a certain point in the game, you can scrounge up weapon repair kits pretty much anytime you need them.