Companies rarely, if ever, release sales figures for digital game sales. Makes compiling charts, and applauding records, difficult! We do know one thing, however: EA's Battlefield 1943 is the most popular console downloadable title ever made.
As part of it's fiscal year report released earlier today, publisher Electronic Arts says that the downloadable shooter has sold over 1.5 million units, and is the "best-selling download-only game on PlayStation Network and Xbox LIVE Arcade".
Since publishers are given raw sales data for their titles, it makes sense EA would know. Then again, without figures for other games to prove it, you're going to have to take their word for it.