FO3:Xuanlong assault rifle
FONV:Fully upgraded Laser Rifle

Well, actually I use sniper rifle a lot in both of the games, too. Especially in FONV with silencer upgrade.
And then there are upgrades for Laser Rifle and even with a scope, make it become much more powerful than the one in FO3. So basically it depends on my characters' SPECIAL, perks and skills. Most of my characters are sneaky type. But in FO3, Xuanlong AR is quite powerful weapon, too. So I use this weapon either in gun blazing way or sneaking upon my targets with sneak critical damage.

In FO3, I would frequently use the combat shotgun whereas in FONV I'd always use either an Anti-Material Rifle or a hunting rifle with all the mods dropped on.

But since I've gotten into modding my game to keep the play-throughs interesting, I've been finding myself using an M4/M16 variant or an ACR

FO3: Chinese Assault Rifle

FONV: anything lever-action or pump-action   

I really only play New Vegas but my favorite weapon to use would have to be the Marksman Carbine (All-American) I guess because its small rifle that has common ammo to find and allows accurate semi-auto shots (I only need one shot anyways )

My DMR, the Ranger pistol, and the Antimaterial Rifle

Back in my vanilla Xbox days, I used a simple Hunting Rifle for Fo3 to literally kill all my enemies. Now I usually go for a 1911 and W9000 in a modded Fallout New Vegas.

I never use the vanilla guns...what's the point with so many great mods to choose from? Generally I like semi-auto scoped rifles, like the Mk14 EBR or the M110 SASS. That's what I carry 99% of the time as I prefer to pick things off at a distance and never get close enough for them to get a shot at me. For close quarters stuff, a 1911 pistol of some kind. Since I play with a custom limited carry weight mod (maxed at 120 lbs) 2 guns, limited ammo and armor/clothes is all I can afford to haul around.

N~dles

Lately I've gravitated toward the M-358 talon.  Its a pretty mean heavy pistol from Mass Effect 3 that is more akin to a shotgun than a handgun.


And when I just don't give a bleep, I break out the M-920 Cain, which usually backfires (e.g. kills me).  Haha!

When it comes to vanilla, I tend to use more Rifles, and Snipers. I never did like Melee weapons as I felt at a disadvantage when it came to enemies being ranged at times. When I get a hold of mods my preferences changed greatly. I use a little of everything with my go-to weapon being pistols cause it makes me feel like a BA.

well if we are talking about the vanilla one i woud say the anti-material rifle with explosive rounds because i so ove seeing every enemy blow up and in the mods weapons categorie i would say the crysis weapon pack because th textur is so amazing

Love my Ruger SR-556
For FO3 it would have to Lincoln's Repeater, no mod weapon really passed it for me.

Antimaterial Rifle with WRP. 

Beretta M92 Samurai Edge Port. http://www.nexusmods.com/newvegas/mods/40284/?
My favorite pistol of all time to use in FNV.

Beretta 92FS one of the best pistol mods out there imo http://www.nexusmods.com/newvegas/mods/50837/?

I use the alot 9mm pistol. The ammo were cheap and abundant in the New Vegas. I never ran out of ammo with the 9mm pistol.

i really don't have a "favorite weapon" but if i had to choose one...it would be assault carbine...but a modded weapon would be mill's M4A1

Millenia Arms - Colt M4A1 by Millenia - Naky:

My Version of AK

Spoiler:

Spoiler:

Spoiler:

I love Heavy weapons with a kick, plus the wood work on the weapons, not metioning that I like every weapon in AK family so I decided to make my monstrum that will hunt every mutant and riders dreams.
muuuuhahahahahahah

    

My last playthrough of New Vegas was done entirely with vanilla weapons. I had never played through the game with all of the expansions, so I went a little crazy when I hit Old World Blues. I think I played through that entire expansion with the Super Heated Saturnite Fist. I just love the superheated weapons so much. They just look so cool. I also like the Bozar and the LAER. Don't really have any favorite weapons from mods. I like Millenia's stuff, but none of the weapons feel like anything that I would like to use for an extended period of time in game. They just seem a bit out of place to me and take me out of the experience. However, I am excited for his Desert Eagle. Gotta love me some Deagle.

The weapon that I have been using the most is the Kar98K by Scottmack, it is a pretty good weapon mod.

Probably the YCS/186 if I was playing vanilla New Vegas. Then Millenia's HK G36K if it was modded.