Fallout 3: it would be Combat Shotgun for the speed and power, Vengeance gatling laser for its damage and jack the ripper for its galery of critical hits.
Fallout NV: it would be Lucky for critical hits and also its prety, Medicine Stick because it hits hard, Riot shotgun, YCS/186 and Mercy because why not, its a great gun if yyou have enought otherwise kinda useles 40mm grenades.

what is your favourite vanilla and non vanilla weapon?
mine is the battle riffle and the HK G3SG/1 from millenia.

EDIT:
merged your topic as it has already been covered in another topic that has been around awhile

:No minus Rep:

falloutveteran20

At the moment i am enjoying a changed stats version of the big boomer. I dont know why but a sawn off shotgun with more power and 1 scatter seems really enjoyable for me. Especially with the soaps Africa armor

It would seem that you missed an existing thread of the same topic. Here is the link for your info:

https://www.gunetwork.org/t6172-what-is-your-favorite-weapon-in-the-fallout-franchise

my all time favourite vanilla weapon is the Chinese assault rifle on my Xbox 360 days my latest save was at 220+ hours and i collected every Chinese rifle i could which i managed to get 36 of from non respwned enemies

for modded id say is the Desert eagle from millenia since i always used his fallout 3 version

FONV version:
http://www.nexusmods.com/newvegas/mods/56745/?

FO3 version:
http://www.nexusmods.com/fallout3/mods/548/?

My favorite for FO3 is the plasma rifle and the non vanilla is the M4 Carbines by metallicalec.

My favorite for FONV is anti-mat rifle cause of the exposive rounds and TOZ-66 by Millenia.

For Fallout 3: it has to be either the Chinese assault rifle or just the normal assault rife, I remember trying to get a Chinese one in anchorage and bring it out of the VR like I did with the combat armour and all the american weapons back when i was on the Xbox sadly I don't think you could.

For Fallout New Vegas: I'd have to say probably the trail carbine or brush gun

For Fallout 3 I'd go with the chinese assault rifle. Regarding New vegas, definitly the hunting shotgun.

I love the hunting rifle (sniper one), pratically the best weapon in terms of damage per ammo, especially if you play as a stealth sniper; Lethal hits pratically kill everything if you use the right type of ammo and you perform an headshot.

(edit) Then, if you want us to choose even modded weapons, I would easily go with the K98 sniper rifle: similar to the hunting rifle but with a nice WW2 touch.

From the vanilla weapons Chinese assault rifle and m4 carbines mods, specially made by millenia.

I was in love with Vengeance from Fallout 3

The Kneecapper. They never see it coming.

"Say hello to the Kneecapper!"

Then I shoot them in the face.

The .223 pistol from Fallout 1, it is powerful looks great and has a big beefy sound when it fires. It is in Fallout 2 but they changed the firing sound so it doesn't as good.

Cowboy repeater, trail carabine and such, those weapons look as great as they are useful !
Also Fat Man... because of reasons !

My favorite would have to be the AER14 Prototype. It's such a great energy weapon and it looks so makeshift that it fits in with the Fallout setting.

My favorite weapon in the vanilla world of Fallout would have to be the good old classic hunting rifle, its sheer accuracy and thrilling headshots would make me scream at my monitor. lol.

Definitely the fat man....love the mini nuclear explosions

From reading this thread I realized that I haven't tried much of the weapons in New Vegas as of now, even with 68 hours of play time according to Steam, which isn't considerably huge compared to some of your playtimes but still.

My current favorite is the riot shotgun. Very fun to get close and shoot the shit out of everyone

i actually love the plasma and laser rifle but my favourite is the avenger yay!

Hmm I suppose i like the Turbo plasma rifle from F1, The speed loaded version of the 44 magnum in Fo2 cheap deadly 1ap to reload whats not to love, The Railway rifle from F3 all aboard the pain train and Greased lightning from GRA in NV that insane speed gotta go fast