Well, you tell me.

Mysterious Magnum (when used by the Mysterious Stranger)

Alien Blaster.

Holorifle.

YK42B pulse rifle.

Target it to the eyes and the enemy will disintegrate.

"Mega Power Fist", by Beat-Co; gives a 20-45 damage punch that doesn't count as a melee weapon ~which means that the PC with the 'HtH Evade' perk doubles their unspent action point bonus to Armorclass at the end of their turn. This can't happen with a gun or melee weapon. Decent damage and makes them very hard to hit.

Guass Rifle. My other personal favorites are This Machine and Battle Rifle.

The Chinese Pistol

Because i'm a hipster.

On a more serious note, i'd have to say: Anti-Materiel Rifle or a good old Minigun

Gauss Rifle. Magnets!

Personally, I kinda liked the fact that Fallout 2 was so tier-based when it came to weapon types. Far more than in any other Fallout game. I remember getting the Shotgun (Model 303?) in Klamath and then upgrading to a Sawed-off, followed by the Combat Shotgun, CAWS, Neostead and finally the Pancor Jackhammer. Atm my favorite Fallout weapon is the...

.223 Pistol

(I called it the Uberpistol, cause it packed a ridiculous punch in Fallout 1. That Gun in FNV wasn't too impressive by comparison, though. But one gotta respect a pistol that uses the .223 FMJ rifle round (M16 ammunition) as ammo haha. The gun is modeled after the one that Deckard uses in Blade Runner.)

Bozar was just a beast in fo2 but the experimental m.i.r.v is unparralalled.