I don't know if there really just one. The comments show most of the results congregate around four modules
B1 Search into the Unknown
B2 Keep on the Borderland
T1 Village of Hommlet
U1 Sinister Secret of Saltmarsh
I think all four are equally good in different ways.
Jeff has an excellent summary in this comment. I especially like the Scoody Doo reference.
Jeff Rient said...
Young or adult players? New or experienced DM? I see at least 4 possible ideal starter modules.
young players, new DM = Search for the Unknown (great advice for DMs)
young players, experienced DM = Sinister Secret of Saltmarsh (every kid needs to learn the lessons taught by Scooby Doo)
adult players, new DM = Hommlet (superior social environ, simple but challenging dungeon)
adult players, experienced DM = Keep (multidimensional, open-form social, dungeon and wilderness)