from fallout 3 I loved Callahan's magnum and Lincoln's repeater most
as for FNV, it would have to be the Gobi Campaign scout rifle.

I never got to finish Fallout 3 (Currently trying to trade some Steam and Humble Bundle games for the GOTY edition), but my favourite weapon from the little bit I did play was the Lincoln Repeater. From New Vegas, it's the Gobi Campaign Sniper Rifle. I have this thing, almost an obsession with named weapons like that. The fact that it also gives a bit of history in it made me happy when I finally discovered it. I always wondered what happened to the person that set up that snipers nest.

For Fallout, Fallout 2, and New Vegas it's the .223 pistol / That Gun. I've always liked the design of the 10mm SMG and the 14mm pistol from the games in which they appear as well. I thought the overall inclusion of more classic weapons was a great addition to New Vegas with the previous mentioned 14mm pistol and the plasma caster.

Fallout: Tactics it's a tie between the Pancor Jackhammer and the Gauss Rifle because they ruin days.

Fallout 3 was the first game where I actually enjoyed energy weapons, and really liked the redesigned plasma rifle. Honestly I'd be hard pressed to find weapons that I don't like from Bethesda's revival of the series.

That Gun in Fallout New vegas Awesome gun use it in every playthrough. And its based on the LAPD 2019 blaster used by Rick Deckard (Harrison Ford) in Blade Runner

Spoiler:

Definitely Lincoln's repeater. Also Laser/plasma rifles, i just love turning npc to ashes.

Definitely the Ranger Sequoia

fallout 3 : Lincoln's repeater and the combat shotgun
NV : anti material, Survivalist's rifle

My favorite weapon is the Marksman Carbine. It looks great, nice rate of fire, and the recoil is nice and vertical instead of bouncy. Just an all around great weapon!

Hunting Rifle! it has a real post apolitical fell and the scope to be honest i haven't found a better mod sniper rifle than the hunting rifle

That Gun, I dunno why but I really liked using it, maybe because of the small LED light on the side that indicated if the ammo is loaded or not ?

Lincolns repeater is my all time favorite.. After that it's the La Longue Rifle or however you spell it.

My favorite fallout weapon is the Chinese assult rifle for fallout 3 and the light machine gun for new vegas.

My fav wep for F3 is Xualong and for NV It's Q-35.

I'm specially fond of vanilla custom-made weapons, so in FO3 the Dart Gun was my favorite. However, I love also prewar objects that are being used as weapons in the postapocalyptic world, so in FNV the H&H Tools nail gun.

My favourite weapon in fallout new vegas is the missile launcher, I like the way it sounds when you shoot it and I like the way it blows up an unsuspecting group of caravaneers a the Mojave outpost all at once

Nothing like blowing the hell out of something with the missile launcher, gotta' agree with you there. But for me, the experimental MIRV just took it to a whole other level.