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.