My thoughts are as follows.....
A weapon chambered for 5.56 NATO. LEO's and Military both use 5.56 and there are MANY civilians with .223 firearms so either of those rounds should be plentiful for a while in a EOFTAWKI situation. Also a 5.56 chamber will digest .223 with only a little loss of accuracy whereas a .223 chamber firing 5.56 will cause unsafe chamber pressures.
The other rifle caliber that I would consider is .30-06. It is one of the most chambered for calibers in America.
.40 cal because it is a popular choice for LEO and civilian use.
9mm Luger since it is a NATO round and is favored by many people. It is also roughly half the weight of .45 ACP. I love my .45 but in a run for the hills situation I could find a better pistol.
My 2 Cents