Well there are a few very strong weapons I can think of, not are all easy to get. There's the Pep Pep, a laser pistol that uses 15 shots into one blast with a damage of 103 or so fully repaired which can be obtained by finding 50 Star bottle caps and turning it into Fetus at the Sarsparilla Plant (excuse my spelling). There's the Multiplasmic Rifle (I think lol) that uses quite a few shots into one blast as well. There are plenty of strong energy weapons at the Silver Rush near North Gate to The Strip. The BoS sell some high powered weapons like the Fat Man, Anti-Materializer, and the Grenade Machine Gun which is located at Hidden Valley. I have not obtained this yet but if you have they Wild Wasteland Perk you can find dead alieans and loot there body for some Alien Blaster, Enjoy.
It's the Pew Pew, that's the name of the laser pistol, I just got it today.
Multiplas rifle, mediocre at best, just like most of the plasma weapons.
The only energy weapons even remotely worth using are the Pew Pew, Gauss Rifle, Alien Blaster (Or the non-WW version), Pulse Gun (A-MAZING for robots), and the plasma caster.
you can buy agetha a grenade launcher in novac, that gun in primm. you can go north at the very top of the map there is a hovering space ship and aliens right under kill them and you get the alien pistol. do east from them you kill these mercs and you can get the gauss rifle or just go into the hidden valley where the Brother hood of steel is and lock pick the door when the guard ask you to follow him say i'm not going any where kill him walk into his room on the left hack his computer on the wall and redirect the turrets to kill all Brother hood member and ahhhhhhhhhhhhh, a whole bunker of equipment from power armor to gauss rifle to laser pistol. oh and word of advise in there shoot the girl in the firing range when shes right up again the window so you can reach her and grab her key so you have access to the armory...