For carrying spare 123A batteries nothing on the market beats the Surefire SC1. Made from see-through impact-resistant polymer the SC1 will keep six (6) 123A safe and dry. It comes with a handy lanyard for easy attachment.
If you need to carry mixed batteries, then your best bet is the Modular Battery Case. The MBC will carry up to 8 different batteries (AA, AAA, CR123A) in any combination. It is made from the same see-through impact-resistant polymer as the SC1 , but it is not as robust as the SC1, nor does it provide a waterproof seal. What it does provide is two easily accessible, easy to separate sections of 4 allowing your OCD to to determine how you want to organize your spare batteries... AA in one section, CR123 in the second or New in one and Used in the Second or red in one and blue in the other.