Monster Hunter Frontier Online, an MMO exclusive to the platform that's made it to the top of the Japanese sale charts. Not out of all Xbox games -- out of the entire country.
The newest installment of the long-running series is a port of a 2007 PC version, beating out several other strong contenders for the countrywide sales figures. Although the Monster Hunter series has been popular in Japan, there's no word on the MMO being released outside of the country. Still, it seems a good sign that you can sell good numbers for a ported game on an unpopular console -- something Square-Enix may well want to consider for the future.