For me it would be a contest between the bolt action rifle, the striker, the riot gun or the mine thrower.
The bolt action has power, and is much more powerful than the semi auto rifle when it has the exclusive upgrade...but it is just too slow and cumbersome...it isn't great for quick sniping...the semi-auto rifle is.
The striker just has no stopping power. Granted, it can be handy against enemies with a shield, but it's lack of power outweighs that I think.
The mine thrower is an interesting weapon (especially if you have upgraded it to exclusive and it shoots homing mines) and you can have fun with it, but it isn't exactly practical. There's so little ammo for it, and it's hopeless if the shot enemy gets close to you before the mine detonates...
Riot gun in my opinion is a waste. Expensive to upgrade, not noticably better than the shotgun (apart from better aiming and less recoil), and both of them are totally inferior to the Striker, if you're prepared to hold on to the shotgun for that long (Ch. 4-1, just before Verdugo).
And while I remember, the Matilda...never understood what was special about this gun either. Wastes ammo ridiculously being the main reason. And takes up a fair amount of space.