AR-15 or AR-10
Shotgun
Glock 19
There is a lot to be said, however, for a .22 rifle in a "survival" situation as you can use it for hunting small game as well as for a weapon. You can store and carry a LOT of 22 LR ammo in a pretty small space and it's light. I'm not a Glock fanboy and don't even own one but I have friends who do and that Glock 19 is probably the most reliable semi-auto I've ever seen.
In a semi-auto, I’d definitely go with a Glock 19. However, if I had to only have one handgun, I’d go with the .357 mag because of the variety of ammo it can take.