I like most the guns, they're all really fun to play around with.
Personally i seem to gravitate toward .357 cowboy repeater (just because i get used to it early on and once it's modded it's not bad later on). the hunting rifle is a beast. But my favourite for large group engagements is the machinegun-rifle or just recently i found Mercy.
If only the ammo for the 40mm grenades was easier to find

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For a pistol eh, i wasn't that impressed with them. But i stuck with a silenced 10mm, packs a decent punch and it looks cool.
I'm about to try a playthrough with a melee/energy weapon heavy character. and i think i'll enjoy the melee weapons.
Lol i forgot C4, remember the exploding pants trick from fallout 3? with C4 this just got fun.
seriously i love fallout new vegas for C4, being able to control when an explosive goes off is so much fun. (sending a deathclaw into orbit has to be my favourite trick)