Kel-tec Sub2000 in Glock 9mm or a Colt M4 5.56
no keltec on the planet will be my ONE gun...I would choose a winchester 30/30 if ammo was plentious or a m4 carbine or a ruger 10-22 in a pinch
I'm going to go with something written by Col. Jeff Cooper many years ago.
A reliable .22 lr with as much ammo as you can carry, and still move about freely, is the ideal SHTF weapon.
.22's are relatively quiet, light weight, and simple to maintain.
Even in a SHTF situation "someone" will have .22's to trade/barter for.
.22's can, in a pinch, be used on deer sized game. (Alaskan poachers use them on Moose).
And lastly, a .22 can be used to "convince" a member of the Opposition, who may have strayed from the main group, to give up his/her larger, more powerful weapon.