While the 9 is the hot thing nowadays with younguns,I think and go by the thinking that a .45 is never a mistake.
Like mentionned above,stash a minimum of 10k primers with whatever powder is needed to replenish 10ks brass in .45acp and you will be well heeled for any(close to medium range) situation that might arise.
Fun thing with good old .45 is that once you've got the brass and if you cast your own bullets,the little beast can be fed for a fraction of a penny more than a .38spl or 9.And a .45 is,well,a .45!