That's why I found it so unusual that so many people--not everyone, certainly--focused on one edition's modules.
Because that "edition" (the leaders seem to be associated with several editions of TSR-D&D, actually) is definitive for so many people? It was basically the game that defined the
role-playing game field in many minds from 1974 through 1999 -- a span of 25 years. WotC-D&D has been around for less than a decade, the 4e version for (I think) less than two years.
So, it's not just "the old stuff" that's favored but "most of the time" by a factor of more than 2 to 1. The oldest modules are perfectly playable with AD&D 2nd Edition! If it is slightly easier to do that than to retrofit a 2e module, that may further skew the appraisal. However you slice it, we're talking the modules that have probably gotten the most play (and maybe the most reading).
Other than mine, I have noticed but one other nominee (Tegel Manor) from the Judges Guild. General quality aside, the JG products just do not seem to have been as widely distributed (and TSR in the 1980s withdrew the license to use its trademarks, which cut that more). Not surprisingly, the JG modules are the only OD&D representatives; TSR's first module (G1) was issued under the Advanced D&D logo even though the core AD&D rules set was not yet complete.
Although my pick -- Paul Jaquays's
The Caverns of Thracia -- was written for the
Original D&D rules set, it was published in the same year (1979) as the first DMG. Necromancer Games published an expanded version for WotC's "3.5" edition in 2004. That's right:
a quarter century after its initial publication, with a very different game in print under the D&D trademark, this was considered (and proven, I think) to have enough commercial appeal to warrant all the work that went into the colorful new hardback volume. People who had never seen it before, who were not even D&Ders when it was last in print, got it and played it and praised it.
The "d20 System" era saw the publication of a lot of scenarios. The sheer volume alone bodes against any one being such a common touchstone as some earlier modules have proven to be. My guess as to one standout would be the "Maure Castle" installments in Dungeon magazine ... the first of which basically recapitulated, and all of which expanded upon,
Mordenkainen's Fantastic Adventure from 1984 (which in turn referred back to a campaign of a decade or more earlier, iirc).
The definition of the
question itself makes it different from "what most represents the way I play" -- today or any day. It is specifically limited to published "adventure modules" (which I, at least, took as counting out "setting supplements", such as
Wilderlands of High Fantasy, as well as more generally informative articles).