N2 the Forest Oracle for D&D is one of the worst I've ever ran. I don't know why the NPcs were not singing or joking but the module makes sure I know they are not.
"A group of seven men approaches. They are following the road east, and are making good time, neither tarrying nor running. Their faces are expressionless. One is dressed as a cleric of some sort, and another is dressed as a traveling drummer. The others could be peasants or serfs going from one location to another for the harvest season. Each carries some sort of weapon. It is plain that they are not soldiers by their haphazard way of walking. They do not seem to be joking loudly or singing as they advance. "
For D&D 3e, there was one called the Standing Stone that:
1) contrived to have the pcs maneuvered into an unjust fight
2) gave the DM advice to ensure that no matter how clever the players were or what precautions they took, the PCs would be fooled.
While I love D&D 4e, basically the entire 9-adventure marquee adventure path - The H/P/E series - is irredeemable trash.
You can mine some good setting bits here and there, but those opportunities are few and far between. (IIRC H2 and P2 are the most workable.) They are giant slogs, just unending strings of loosely-connected combats, and they contributed to 4e's overall poor reception. There's a few good setpiece ideas here and there, maybe, but ... ugh.
They showcase the worst the edition had to offer, and therefore fill out the top of my own list.
There was a 2e Ravenloft module named "Thoughts of Darkness" that still holds my record for most disappointing adventure purchase. Feast of Goblyns was so much fun, so I eagerly bought this. I got a contrived plot, no subtlety or horror, poor flow, and a freakin' ton of 14th lvl drow and mindflayer vampires.