Are you wanting to solo NCR Army by bursting at 1-hex? Or by using crits? How were those guys you saw doing it? How do you prefer to do it? There's so many options.
First thing i do is compare the guns on wiki, and remember to compare the ammo too because a gun alone is not the full picture - it's a gun + its ammo that should be considered. (Stalker's M60 does so well because 7.62 ammo is so awesome.)
Once you choose your preferred weapon, then study the perks. (Quick Pockets is good for one gun i know: rocket launcher.) Use a planner to verify how best to level a build. Best thing is...there is no limit to registration, so try one, practice, try another one, get some more practice, keep trying and develop better understanding of the mechanics.
So you tell us...which gun do you want to use the most?