"zombie apocalypse".... Wow, someone's got quite the creative imagination on them...
To give your gun-porno-daydreams some substance, in my opinion it comes down to "calibers", more than specific firearms, to ensure that you have a firearm thats compatable with the ammunition which you are most likely to come across.
A person can only carry so much ammo, and last time I checked, zombies don't go down easy... Not to mention you also need to carry food, sleeping bag, medical, etc... One could have the most reliable 10mm Sig pistol out there, but once your outta ammo...it's a paperweight.
.22, .223, .308, 30-06, 9mm, .44 mag, .45, .357/.38 and 12ga are the most popular...
The last thing you'd want to look at is "what happens"...if China decides to invade, then adding 7.62 Russian to the list is a good idea. If its martial law, rest assured the gov't will be after every last 5.56mm round they can get their hands on...
30-06 is a great caliber because nearly everyone has one and the father away one is from the firefight/danger, the better then chance you get to live to see another one... .44mag is another because one shot, someone's going down, and in a LA rifle, you know u wont blow throug rounds and live a lot longer. If you start looking at pistol calibers first, your getting waayyy too close to the action to expect to survive long...
Just my two cents, but seriously, threads like this are meant for some other website. To paraphrase Deciderius Erasmus "War is delightful to those who have had no experience of it."