Alien3 - Both Ridley Scott's original and James Cameron's followup were classics of science fiction horror. David Fincher killed the momentum in one fell swoop. Looking back on it now, it actually has many redeeming qualities, but it still managed to annoy fans and turn off newcomers right from the beginning.
Back to the Future Part III - Personally, I actually enjoyed many scenes from this flick more than I did most of Part II, but setting it in the Old West was a bad choice, abandoning the nostalgia of the first and the futuristic wonder of the second, creating a big disconnect from the rest of the trilogy.
Batman and Robin - All of the Batman movies up until that point had their flaws, but this one totally demolished the franchise critically and at the box office (until Christopher Nolan saved the day by reviving the hero in his Dark Knight trilogy). Joel Schumacher's previous batflick Batman Forever seemed like Citizen Kane compared to this bomb (okay, that's a huge stretch, nothing can make Batman Forever seem like Citizen Kane, but I hope you see my point, Batman and Robin was one of the biggest letdowns in motion picture history.)
Blues Brothers II - The original with John Belushi and Dan Ackroyd was one of my favorites of all time. The long delayed sequel was horrible and should have never been made if they couldn't do it right.
Book of Shadows: Blair Witch 2 -- This was nothing at all like the original, which became one of the most profitable movies ever produced. The Blair Witch Project was unique and started a subgenre of faux "found footage" films, but the sequel was just a derivative mess.
Chronicles of Riddick - Pitch Black was a wonderful surprise, a tight, suspenseful, thought-provoking, well-produced little gem. Like so many others, I wanted to see more of Vin Diesel's character, but the sequel was underwhelming. A new chapter is hitting theaters this year so let's hope it's an improvement.
Escape from L.A. - John Carpenter's Escape from New York is rightfully one of the best cult movies of all time from start to finish. This sequel was an abomination. I want to erase the vision out of my mind of the anti-hero Snake Plissken surfing.
Exorcist II: The Heretic - The original is fantastic, one of my top ten favorites. The sequel, not so much. Actually, Exorcist II is probably one of my least liked motion pictures ever captured on celluloid.
The Godfather Part III - It's tough to live up to the expectations of the first two films, each of which won Best Picture at the Academy Awards (yes, even the sequel won Best Picture). This third part was disappointing on many levels with a number of flaws that made me wish Francis Ford Coppola would just pretend Part III never happened and try again.
Halloween III: Season of the Witch - The first two flicks were about the murderous Michael Myers, so you knew this would be a chore since Myers only had a blink-and-you'll-miss-him cameo on a TV monitor in one scene.
Hannibal - One of my favorite horror films is Silence of the Lambs and this continuation of the story was a weak attempt to stretch out the franchise (which is still being stretched).
Highlander II: The Quickening - How do you build on the success of a cult classic sleeper hit? Not by turning the plot into some convoluted mish-mosh.
Indiana Jones and the Kingdom of the Crystal Skull - The long wait for the fourth episode in the Indiana Jones adventures was a huge let down. It also coined the phrase "nuke the fridge," which is the move version of TV's "jump the shark" when a series hits the point of no return.
Matrix Reloaded - The first film was a true favorite, a groundbreaking sci-fi masterpiece. The sequel was a big, fat dud.
Ocean's Twelve - I confess, I never saw this sequel, but so many people named it as "disappointing" that I had to add it to my list.
Rambo: First Blood Part II - Sylvester Stallone turned his poignant John Rambo character into a caricature.
Rocky V - So many people name the third film as disappointed, but Rocky III still remains one of my all-time guilty favorites. The Rocky flick that I found to be the most disappointing was part five. The heart was in the right place, but the execution never lived up to my expectations. The story was ho-hum and everything felt anti-climactic.
Speed 2: Cruise Control - The original made Sandra Bullock a superstar and turned Keanu Reeves into an action top-liner. The sequel is a forgetable snoozefest.
Spider-man 3 - Sam Raimi did so well with the first two Webslinger superhero movies. Then the studio interfered and forced him to add the villain Venom into this third part, and the result was a bloated jumbled disaster, leading to a quick reboot.
Star Trek V: The Final Frontier - The very first Star Trek: The Motion Picture was a boring disappointment, but then I really enjoyed the next three movies, which have been dubbed "The Spock Trilogy." Then came Part V, terrible on so many levels.
Superman III - The first two Superman movies are beloved, the third one is laughable in all the wrong ways.
Terminator 3: Rise of the Machines - Although I love Kristanna Loken, this third film failed to recapture the heart and imagination of the first two.
X-Men: The Last Stand - Bryan Singer delivered two excellent motion pictures, but then Brett Ratner took over and killed off a couple of the fan-favorite characters, not to mention delivering a mediocre story.
Those are the biggest sequel disappointments in my eyes. What do you think?