My current go-to is a heavy elite frame, tesla core upper with a precision tip, and an unmodified plasma core in the lower. The primary fire hits like a truck (a well-aimed shot can one-shot many necromorphs on Impossible) and the secondary is great for crowd control. My only issue is the slow rate of fire and wind-up time on the upper, as well as the small clip size. I find it really useful for those sequences where you're absolutely mobbed by necromorphs, but it's very often game over as soon as I need to reload. Another minor issue is that after you fire the upper, any additional fire attempts before the weapon has been "cocked" again don't register, so I sometimes think I'm charging up the weapon for a second shot, but Isaac is actually just sitting there looking stupid.
[Edit: I think I got the upper components wrong, so I corrected that]