I'm going to have to go with my M14 for this one. If I had an AR-10 type, I might go that way but since I don't, this is it. I have multiple stocks, a pair of scope mounts, lots of magazines and ammo, and lots of trigger time and experience with this gun. They also have the reputation of being extremely reliable. While 68Charger's thought on the easily concealable handgun has some merit in my opinion, in the end I feel that a pistol is something you use to get yourself a rifle.
Of course, I can't imagine myself only having ONE gun in a SHTF situation, unless it was just to get back home with. There's something basically wrong with only having one gun.....