In terms of the hype, I'd have to say "Last Tango in Paris" was a stunning disappointment. It wasn't sexy, it wasn't acted well, it wasn't shot well, the story was meandering, and the characters were uninteresting. The only thing it had going for it was the cute French girl's big messy '70s Penthouse bush! Marlin Brando was terrible and his character was a complete weirdo. I know, I know, some of you out there are probably thinking it was the best "American weirdo rapes a French girl and then has all kinds of weird sex with her" film ever made, but trust me, if you haven't seen it, it's a complete turd!
Agree on Last Tango AND Godfather. One was just poorly done; the other, just another gangsta movie about Mafia. There had to be 1000 of 'em in the 70s.
As two other posters have said, "Titanic" bites the big one.
The only other movie that was that bad was "Shock Treatment", the sequel to "Rocky Horror Picture Show".