A strong candidate seems to be B2: The Keep on the Borderlands, by Gary Gygax, first published in December 1979.
Lawrence Schick, in his 1991 book Heroic Worlds, describes the
adventure as "A good start for new players" and speculates that at the
time, there had probably been more copies of B2 printed than of any
other role-playing scenario.
Looking for the exact quote, I found this Google site called Zenopus Archives:
"There probably have been more copies of B2 printed than any other
role-playing scenario" - Heroic Worlds by Lawrence Shick, 1991 (pg
"The total print run for B2 is easily in excess of a million and a
half units" - "Looking Back" by Ryan Dancey in The Story of TSR, 1999
The Acaeum website, which I didn't know until today, but seems to be a reference for Wikipedia, has a Print Run Estimates page about D&D products, which again puts B2: The Keep on the Borderlands on top, with two runners-up:
B2 Keep on the Borderlands: 1,000,000+. Source: 1999 Silver
Anniversary Retrospective booklet
S1 Tomb of Horrors: 250,000+. Source: Gary Gygax verbal comment
S2 White Plume Mountain: 175,000+. Source: WotC employee
There's still the problem of comparing print runs with total sales, specially in the case of B2: The Keep on the Borderlands because - although the module was still sold separately - it was included in the Dungeons & Dragons Basic Set from 1979 to 1982, just before the famous 1983 Red Box edition.