The Avatar Trilogy that bridged 1st and 2nd edition Forgotten Realms were terrible as adventures. Quite literally the only time the PCs get to do anything is during random encounters. Several points in the adventure flat out state, "The PCs should not be able to effect these events," and there is a very stupid forced capture in there as well. The PCs get to witness climactic events such as Mystra dying and Elminster fighting Bane, but don't actually get to do anything during what are essentially cut scenes. At one point in the Shadowdale module, there is literally a span of 3+ pages of pure flavor text, during which the PCs get to do nothing but stand around and watch. And the ultimate reward for getting through the whole trilogy? Watching a bunch of NPCs become gods.
Other modules have lame plots or stupid design, but I don't think anything can be quite as un-fun as going through the Avatar trilogy as written.