Sonic Adventure 2 Battle isn't just a top-seller for SEGA in general. It's also one of the best-selling third-party games on Nintendo's Gamecube, selling nearly 500,000 copies in the US in the year or so since the console's been available for sale.
The news comes from IGN, which compiled sales data on the Top 10 games over the last year. Besides Nintendo's own exclusives (which to be fair, you'd expect to top the best-selling lists of their console), Sonic Adventure 2 Battle comes just after Star Wars and comes fifth overall.
See the full chart below. Great news for Sonic and Sega!