IIRC, BECMI was actually the most sold and most translated version of D&D up to the 5e. I"m not sure how 3.x. D&D would look in comparison if you stacked all Pathfinder and d20 games to its numbers.

The most influential is easy, though: it's original D&D, because it codified modern fantasy roleplaying games and all versions after it, several other tabletop RPG franchises, novel franchises and videogame franchies can be traced to it.