Saiga, followed Closely By USAS, the pump action ones make you feel like a badass, but are terrible at actually killing fools.

Hate using Shotties in general so don't have a favorite. but I guess USAS is a good one. Used it to protect an MCOM and it helped alot!

Saiga with slugs. It's like a sniper rifle without any bullet drop. Outstanding.

The one between my legs... WHAT UP!!! 
 
I gotto go to bed...

I love the SPAS-12, just because it is the most badass shotgun in the game :P.

SAIGA and NS 2000 are the only choices.

SAIGA and Remington 870.

870 with or w/o slugs, starting to use w/o recently it's still a beast on hardcore. 
Also because of using it without I got my gold star with my M9 :P

 SAIGA
Pump action ones are a bit unreliable since if you accidentally miss or are not close enough you are usually screwed  as the other guy will off you before the next round is pumped into the chamber. I usually only use shottys to assault Mcoms as Recon since after arming and placing the C$ you can throw down a motion mine to watch for dudes coming to disarm. when they come close use the C4, then if more come use shotty to clean up.

spas-12

I find that all the shotguns work for me, but after taking out one guy it's hard to get his buddies. I prefer the 870 for close up and saiga 20 for a bit farther. Once I get slugs I plan on sticking with the 870.

40mm shotgun under my rifle or one of the pump action ones with slugs.

I'm a big fan of the 870 I just love using it with Slugs and Magnum ammo.

Neostead = best stats, worst looks.  
 
And BFBC2 is screwy like that; a single bullet to the head should incapacitate someone even if they're wearing armor, in BFBC2 it's like 3-4 :/. Even a shotty takes more than one headshot sometimes. 

" Neostead = best stats, worst looks.   And BFBC2 is screwy like that; a single bullet to the head should incapacitate someone even if they're wearing armor, in BFBC2 it's like 3-4 :/. Even a shotty takes more than one headshot sometimes.  "

870 combat. I feel so bad ass when I get a kill with it.

I likes the SPAS with slugs. Headshots abound.