My favorite modules are different from what I consider the defining modules to be:
Basic D&D: Keep on the Borderlands, Isle of Dread
1st Edition (and earlier): Against the Giants, Village of Hommlet, Tomb of Horrors
2nd Edition: Ruins of Undermountain, Night Below
3rd Edition: Sunless Citidel, Forge of Fury, Return to the Temple of Elemental Evil
To me, 'Edition Defining' adventures are the ones that everyone knew and probably played and/or represented the iconic experience of that edition. When 3e came out, Sunless Citadel was THE module to play...being the only one available helped.
When 1e hit the stores, Against the Giants and Hommlet were the modules you saw sitting next to the books. When you bought the Red Box, Keep on the Borderlands is what you got. These were the modules that defined the editions.
Much as I like modules like Shackled City or Ravenloft, they were came later and redefined what the edition would be or reinforced it. But they came along well after the game was laid down. Ravenloft and the Desert of Desolation series, for example, changed what people expected from D&D. They may have been the fondest memories for that edition to some, but they didn't define it, IMHO.