Well, North America is definitely the largest market, so it provided the most sales, but per capita I have heard, anecdotally - mind you, that Golden Sun is the most popular both in Europe, and also seemingly in Latin America. The few Smash polls conducted regionally tend to back this up, but I don't have any on hand, unfortunately. Plus he gets a different name in basically every language... so that's fun to keep track of.
Following those two general regions I guess the US & Canada would be next, with Japan trailing noticeably behind. Though not to the extent he won't show up on popularity polls there, he'd just be lower.
(Sorry Aus & NZ & everyone else, no idea about you lot)