Some metal armors give good resistance to energy weapons.
The Awareness perk can show you the different damage resistances of different enemies. So if you see something with low energy resistance, use an energy weapon. Same with Ballistic resistance. If you see something with really high resistances all around, try something with armor-piercing capabilities (later ranks of the Rifleman perk add armor-piercing on all rifles). For some other enemies you're better off just focusing on the legs to immobilize them instead of trying to face them head on, Feral Ghouls especially. For most robots you're better off trying to disable some of the limbs, too, although you don't want to get rid of all of their arms or they'll run up to you and self-destruct.
Enemies like Mirelurks have specific weak-spots. You'll have a hell of a time trying to shoot through a Mirelurk's shell, but if you can hit their face they go down a lot faster.
I never do. Just make sure you have the best receiver, the appropriate perks and suitable sights, and try to place your shots. Lasers I found underwhelming, but a suppressed ballistic weapon lets you see what you are shooting at and provides the fastest recovery. It's excellent for shooting the mini nukes on those pesky supermutant suiciders.
i have the opposite experience with the sniper barrel on laser rifles, who gives them heavier hit than the .50 bullet but with very high rate of fire and low recoil.
Suppressed ballistic has the benefit of not giving away your position.
In the case of people the armor they wear determines resistances. Like your armor, the specifics can vary, but in general:
I hope this helps. Good luck, and good hunting.
I have combat rifle chambered in .308, the rifleman perk maxed out and the Killshot perk from MacReady. I find that a default tactic of shooting everyone in the head works well enough that I don't bother carrying a bunch of different weapons.
For mutant suiciders I've taken to shooting a leg first thing, as soon as I see them. They're a lot easier to handle when they're limping