Since I am secure in my dwelling, and have no intention of "bugging out," obviously my entire collection is my "go-to" SHTF gun. Happily, I won't have to make agonizing choices that might cause me to abandon lifelong favorites because they might not be as "tactical" on the road or in the woods as other firearms might be.
What I grab first, of course, would depend on the nature and urgency of the situation. All things being equal, and generally speaking, a high-capacity tactical carbine would be first in line. BTW, why do you specify a pump-action shotgun? Don't you like autoloaders?