The ISA shotgun from Killzone is neat, but the reload time (between rounds, and when loading new shells) is quite long. The secondary fire option - both barrels at once - is guaranteed destruction for nearly anything in the way, though.
I also like the Helghast assault rifle - a 30-round automatic rifle, with a single-round shotgun barrel beneath the main barrel. Great for surviving close-quarters ambushes, when regular automatic fire alone isn't enough.