I primarily use a variety of millienia's weapons , but usually i go with the colt pocket revolver and the m4. In addition to this i usually stray away from using energy weapons and any other form of fire arm and melee weapons i don't bother with because i only like the katana however i always think it would look wrong on an NCR vet for some reason.

I use hunting rifles and c4 mostly. hunting rifle because its not too hard for me to find or buy and c4 because its fun to use. Nothing like lining up a whole bunch of c4 on a street and blowing someone up sky high!

I also use millienia's weapons mostly the AK47, but besides that i like using the light in shinning darkness with a silencer.

I used a moded m4a1, laser sight, vertigrip, eotech holo, handles super nice and goes well with my armour mods.

If we're going vanilla, then I would use the assault carbine with all it's mods.

Vanilla Fallout 3: Vengeance gatling laser.
Vanilla Fallout New Vegas: Hunting Rifle
DLC Fallout 3: I use the cut melee weapon 'spanner' There's just something fun about using a backwards spanner in combat. The way it's held reminds of a sonic screwdriver for some reason...
DLC New Vegas: Bozar. That thing is just awesome.

Mods: any G36 style weapon. Also Millenia's weapons from time to time.

I like the hunting shotgun/Dinner Bell. It can fill the role of close, medium, and long range combat. And with GRA, I can load pulse slugs and cut down on robots easily too.

I rarely found myself using a vanilla weapon, considering a replaced most of them. I assembled this sexy AR-15 in Nifskope tho. It has one of them holographic sights with the magnifier, PMAG, and the angled fore-grip. One nice rifle for my character.

Back when I used vanilla weapons I used the marksman carbine. It had a cool look, it was semi-automatic, and it had a scope.

Knifes and 9mm pistols, mostly.

yea i rarely use a vanillia weapons as well there are way too many good weapon mods not use out there but i usually use a m4 and a sniper rifle like the barret 50 cal my favorites

Doesn't matter what rifles I try I always end up going back to my trusty ol' hunting rifle in new vegas anyway. I never really played a modded Fallout 3 so i can't really say for that game.

In fallout 3 it was the xuanlong assault rifle. I'n new vegas i switched weapons so much it's hard to say but i believe the weapon i used the most would probably be the bumper sword when i was doing my melee character run through.

I use russian assault rifles,for example:AK-74M,AN-94,AEK,AS VAL,or sniper rifles such as SVD,SKS,SV-98 and others

For vanilla FO3:  xuanlong assault rifle.  Although I also carry a silenced pistol, sniper rifle, combat shotgun, and a missile launcher.  Always need the right weapon for the situation.
For modded FO3:  no particular favorite as I like to try new weapons as soon as I load a mod with something interesting.  Just my Opinion!!!!  Cheers!!!  V/R  MistTiger

The weapons I've used most frequently in the Fallout games would probably be anything resembling an M4. Whether it's an HK16 or the Assault Carbine, I grew attached to a militaristic styled character as well as weapons. Thus why I couldn't stray from my trusty HK17 Sniper Rifle from the Shrapnel mod!

the vanilla weapon I use the most in New Vegas is Lucky, fallout 3 I use combat shotgun the most.
far as modded weapons go the Steyr AUG A1 is my go to weapon.

As far as Vanilla goes easily the Marksman Carbine and even with modded weapons I prefer weapons that like the Carbine are semi-auto and have a scope normally an AR-15 style weapon.

I usually use HK416 pitbull and BOSS M249 from Book of Earache mod and the M60 from 20th century weapons mod. although the M60 from that mod easily gets broken so better pack alot of scrap metals in a way I don't end up getting overweight.

I use a modified hunting shotgun, with M4 stock to increace accuracy.