I have a list of forbidden weapons that I just refuse to use in the respective game it belongs to. The silenced M9 hand gun in CoD4 is definitely up there. The M4 Carbine from MW2 is another one that just doesn't have the ability to kill people. In Fallout 3 I hated the Dart Gun. I just didn't see the point of it. Once I got to a level high enough to actually encounter deathclaws regularly I had already gotten the Alien Blaster and could 1 shot them most of the time, and then rarely it was a 2 shot. And besides deathclaws, there are really no enemies that would sprint at you especially quickly except for behemoths, but the Dart Gun didn't work on those guys anyway. I always hated the Spartan Laser in Halo 3, mostly because I don't even own a 360, so whenever I would play I would suck with the controller, suck at the game, and I could never line it up right so it was basically just a glorified laser pointer at least in my hands. Any sniper in Black Ops from what little I played of that piece of garbage was useless. You couldn't hit people and when you finally did it was basically exclusively hitmarkers, even with the sniper that was supposed to be the strongest.
These were just the worst weapons in my opinion from what games I've played, and I'm probably missing a few.