Usually three types of movies appear on these lists:
1) Movies that are truly terrible in every respect (Manos: Hands of Fate)
2) Movies that actually do have an interesting idea but awful execution (Star Trek V)
3) Movies that aren't actually terrible, but rather terribly disappointing (The Phantom Menace)
So, in reality, there are WAY worse movies out there than Star Wars 1,2 and Matrix 2,3, but these films are at the top of the list for most disappointing films of all time. To me, that's a different list (and none of those were as disappointing to me as they were to the general audiences).
Anyway, some truly awful, awful movies in nearly every respect are:
Manos: Hands of Fate
Batman and Robin
Santa Claus Conquers the Martians
The Return of Jafar
Drop Dead Fred
Here on Earth
Mortal Kombat: Annihilation
The Perfect Storm
Robin Hood (1973)
Superman IV: The Quest for Peace
Teen Wolf Too
They Call Me MISTER Tibbs!
Weekend at Bernie's
The Star Wars Prequels and the Matrix Sequels mop the floor with these films.