Shotguns, every damn time. Nothing in gaming is more satisfying than flanking an enemy squad in BF3, then opening up on them from about 2 metres away with the M1014, and dropping the whole lot of them in 3 seconds flat.
Shotguns are more than just running around and shooting people in the face from point blank. You have to know the areas of maps you can safely use, angles, corners, and ambush spots are so much more crucial than with other guns, due to the fact most shotguns in MP games are pretty much castrated.
One of my favourite in game shotguns is the Olympia double barrel from the first Black Ops. So, so much fun, and only having 2 shots is a nice added challenge.