This being posted from the GalCiv2 forum, it wouldn't be right if there wasn't a mention of Master of Orion 3 being a total letdown to the previous games .
Games: MOO3 (unfinished); XCOM Apocalypse (maybe a private opinion);
Movies: StarWas Episode I Phatom Manace (by far the biggest disapointment); Matrix 3 (They just went tooo far); StarTrek 9 - Insurrection (Boy!! - bad episode disguised as a movie); Next Karate Kid (unnecessary);
Agreed. For PC games, none was ever a bigger disappointment than MOO3. Rome Total War comes in second, although at least that one still has some charm & appeal -- MOO3 was simply broken.
As for movies, I agree with greenpower's mention of Matrix Revolutions and The Phantom Menace. (I would also add Attack of the Clones, as the writing/dialogue is about as craptacular as it comes.)
I didn't find Star Trek Insurrection so horrible, but Nemesis (the 10th movie) sucked *hard*. Only the first Trek film (The Motion Picture) was worse IMHO.