I'm a huge fan of both Of Sound Mind (Woot!) and Banewarrens, for the same reason: ultimate customizeability. Both give a lot of guidelines for what might happen if the plot goes off track, and Of Sound Mind even gives suggestions for changing the entire premise and setting of the adventure. Banewarrens is almost as easy to change around, but much longer. I can't really pick between the two, but I'm only half way through running Banewarrens so far. The Freeport Trilogy (less Hell in Freeport, which I dislike), is also great, and full of flavour. Finally, pretty much anything from Dungeon Magazine is going to be a good mod, I've been extremely impressed at the continuing quality.
--Seule