Which weapon is most effective with the Assault Shield ability?

So the Assault Shield ability modifier is described as the following,

Equips a shield that grants the user's sidearm increased damage, knockback, increased accuracy, and faster reload speed.

Which side arms will gain the most benefit from these bonuses?

Thinking mainly about comparing a shotgun, large magazine pistol and a slow firing small magazine pistol. Better damage would mean that rate of fire is most relevant? But a bigger hit from, say, a Magnum might prove more effective.