This was a tough one. Most of my candidates for "favorite published adventure" are old because I don't buy a lot of published adventures anymore. I've had a lot of fun with Forgotten Temple of Tharizdun and Against the Cult of the Reptile God, especially. I guess I'm going with When a Star Falls, though.
There are just so many cool and fun-to-use things in this one, from the wilderness romp to the freaky memory fungus to the whole deal with the group of astronomer/sages and their political maneuverings.
Most of the "UK" series modules were, at the very least, interesting. Sinister Secret of Saltmarsh is another of my favorites.