Campbell
Relaxed Intensity
The reason is Tom Moldvay. Not only was his edit the most consistent and clear text with straightforward play procedures - he also was responsible for many of modules that are held up as a gold standard in the OSR community (Castle Amber, Isle of Dread, The Lost City). His approach to refereeing and adventure design (that is concerned with being both challenging and mostly fair) are foundational to the community.
To a fair number of people in the OSR community Moldvay is D&D.
To a fair number of people in the OSR community Moldvay is D&D.