Old Edition:
B4 The Lost City
U1-U3 Sinister Secret of Saltmarsh Trilogy
N1? Against the cult of the reptile god
Under Illefarn was also good, but I recall it took a little work and development. It was fairly huge.
*I thought Castle Caldwell was one of the all-time worst adventures ever written until I read some of the "Marco Volo" crap in 2E D&D.
3.x Edition:
NeMoren's Vault by Fiery Dragon. (top notch, I started 3 campaigns with this adventure).
Lost City of Barakus by Necromancer Games. (This is a good mini-campaign, but there is so much to it you may have to scale back XP.)
Honorable mention:
Crucible of Freya by Necromancer games... the core adventure is OK, but what makes it great is the free supplemental stuff, including a blind crone, a bipolar etin, and other awesome stuff.