For me, it would be the ColecoVision or Atari 7800. VGChartz lists the 7800 as 4.3m sold. I've never seen a concrete lifetime sales # for ColecoVision, but I've seen it estimated anywhere between 2m-6m. I believe this is because the last reported figure was well before the video game crash of '84 in NA and the console was still selling for quite a while after that report was made.