Quote Originally Posted by Vahnavoi View Post
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.
Those number might be misleading depending on how it is being counted. BECMI is extremely popular and its influence should be recognized, true. Very true even, I don't want to diminish how valid your underlying point is, the following is just to contextualise it a little.
To build on my earlier comments when I said '3/3.5e were good at building loyalty' was because when each successive iteration of D&D came out, a large portion of the playerbase would transition with it to the newer systems, supporting them in their growth. 4e was resisted by a large enough section of the playerbase not transitioning that provided the need for pathfinder.
Also in terms of sales, there's also a large portion of the playerbase that buys into multiple systems. BECMI has a very solid reputation, and people buying later editions will tend to eventually buy into that thanks to its legacy status. However, a large portion of that are players already inside the tabletop community, working their way up from older editions, or going back and revisiting them for the nostalgia/legacy experience.
For 5e, it is the first system of a substantially large portion of the current player base. True there is a big portion that transitioned across from older/alternative systems, but there's still a lot of new blood introduced thanks to 5e. Again, as KorvinStarmast has said, 5e was growing out of fertile ground, where as those old-old edition had to build up from nothing. Still, the underlying point would be we need to look at the growth to the playerbase population based on people's first system they started on rather than raw sales numbers of each edition.

Quote Originally Posted by Vahnavoi View Post
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.
Very much so, in the same way tolkien set the tone for fantasy in general, or how starwars cemented the formats for blockbuster cinema.