Well, I started out as as OFEC. Then, I realized that countries changed so much throughout their histories, i.e. invasions, occupations, gov't changes, etc. Most of those changes had corresponding unique coinage.
Since it's pretty much impossible to have a coin from every single country and every single period in its history in the world, I've focused my collecting on European countries, particularly Eastern European. Ideally, I'd like to have at least one coin from every European country (including cities, states, territories, etc.) from the Middle Ages to the present.
In terms of personal preference, I love Russian, Dutch, and am currently becoming interested in Livonian coins.