Since I like too many guns to count I might as well go with the gun that has saved my butt in a lot of situations. That would be the humble 10mm Pistol. It has a 12 bullet clip and is the first weapon you get. While the other weapons do have more power and better clip sizes the first 10mm Pistol I got would preform miricals for me. I had low health, no stimpacks and saw an enemy with full health about to finish me with a grenade. I entered VATS mode and saw I only had a 1% chance of hitting the grenade and used all my 4 shots since it was my last hope. First bullet hit the grenade and killed the enemy which allowed me to take his weapons, armor, ammo, and other stuff, fast travel back home, sell the loot, buy the workbench for my house and heal without having to do everything over again.