I found it a tough poll because some choices might be remembered for a long time, but aren't necessarily "great adventures."
For example, I briefly played in Tegel Manor back when I began D&D (not really sure why we only went a few rooms in). I remember the map vividly and spent a long time getting a hold of a copy.
Looking at the adventure now, it's not very good. Even for the time, I don't think it's good. On the other hand there is a LOT of bits hidden in the adventure that are excellent. I fully intend to use Tegel Manor in my campaign, but not without an almost complete overhaul.
Regardless of how good it is, I expect the World's Largest Dungeon will be remembered for a long, long time. I wouldn't be suprised if it commands very high prices 20 years down the road.
With the exception of the Shackled City Adventure Path, this list ignores the many fine adventures seen in the pages of Dungeon magazine. While there are many that I thought were fine adventures, I'd definitely put in a word for the following:
Rana Mor (#86)
Glacier Season (#87)
The Seventh Arm (#88) Loved it!
Kambranex's Machinations (#91)
There was also a great drow-based adventure by Monte Cook, the title of which escapes me at the moment.