really just depends on your preferences. I'd recommend playing through the campaign to get an idea what kinda weapons you prefer (of the pdr's and shotguns, which i think is the two primary weapon types of engineer)
the scar, bar none. bring it over to bodark and over gas it. give the saiga a try too, its incredible if you've got a rifleman with some tactical re-supplys. its a shame that treyarch absolutely castrated such a brutal gun in black ops 2.