Batman and Robin - I don't know how anyone at any point could perceive anything (except for Alfred 'dying', that had potential) about that production as a good idea. It's awful on every conceivable level.
Jaws 4: The Revenge - I get why it was made, even though the franchise was in decline it was still profitable. But at some point during the production someone should have realized 'hold the phone, there's a roaring shark here...that explodes for no reason when it get's stabbed by the boat. The shark can catch up to a plane. The main character has flash backs to events she was never a witness to...maybe we need to work on this a bit more'. But nope, it happened, and it was awful.
Superbad - the only time I've ever almost (and should have) walked out of a movie.
Most disappointing: these aren't 'worse' than the ones above in terms of being in general awful, but they're around the same level of disdain because they aren't as good as they could/should have been (or they weren't what was advertised, or they didn't live up to their potential)
Spiderman 3 - Except for JK Simmons as J Jonah Jameson, I hated every thing about this movie. As Spiderman and Venom fan, it ticked me off to no end.
X-Men 3/Origins Wolverine: Both flicks suffered from an over-abundance of shoving mutants in just cause they could, changing the characters in ways that were a disservice to fans (eg Gambit, Cyclops, and especially Deadpool), and sacrificed story for action pieces (in films with mutants action pieces are necessary, but shouldn't be there just for the sake of being there).
Iron Man 3 - outside of RDJ's performance, I hate this flick. Not at all what we should have gotten. Hugely disappointing. Look at the tone from that first trailer - THAT'S the Iron Man movie I want, that's the Mandarin I want.
Prometheus - Here's a flick that I can enjoy for what it is, but I'm still disappointed that it's not what it should be. I swear, Ridley Scott became so adamant that this wasn't a straight up prequel to Alien that it became a detriment to the movie. Everything about that movie screams Alien - and if you change a few things, you still have a movie that establishes it's own story to spin off from while being a prequel (and a rare excellent one) . But now if the studio ever decides to do an Alien prequel, we'll have a movie that has to tell almost the exact same story as Prometheus. Makes no sense.