The PSP's best-selling title in North America wouldn't make the Top 5 list for Nintendo DS.
According to sales data provided by NDP Group (thanks to Game Informer and Gamasutra) the best-selling video game for Nintendo DS in North America is New Super Mario Bros., with 4.5 million units sold. In comparison, the PSP's best-selling title, GTA: Liberty City Stories, has sold two million units.
Top five best-selling DS games - Nov 2004 - Oct 2008
New Super Mario Bros. - 4.5 million
Mario Kart - 3.5 million
Pokemon Diamond - 3 million
Super Mario 64 DS - 2.7 million
Brain Age - 2.6 million
Top five best-selling PSP games - March 2005 - Autumn 2008
GTA: Liberty City Stories - 2 million
Midnight Club 3: Dub Edition - 1.1 million
Need for Speed: Most Wanted - 1.1 million
GTA: Vice City Stories - 0.95 million
Star Wars: Battlefront II - 0.809 million
According to Game Informer 20 DS titles have reached sales of one million units or more. In contrast only three PSP titles have reached sales of one million or greater.