I would say the shotgun for getting out of a sticky situation.
Lots of zombies....
The magnum for bosses, but when you put the parts on i felt it lost some of its appeal, really just because of the recoil which sometimes was a pain in the ass!
The machine gun was badass, it helped me out a lot in the 2nd scenario when you go into the room with three of the hunter type monsters to get the master key?
I died at first and when i brought that beast with me the next time they had no chance of even getting close!
Aw sheea!!!