Teenwolf 2

Dumb and Dumber When Harry Met Lloyd (a prequel, but it was awful)

Caddyshack 2

Major League Back to the Minors

The Hangover 2

I'll add 'White Noise 2' to the list.

Home Alone 3

there should be a cavet that the sequel has to be something that a studio intends on doing well and can't be some B.S. straight to dvd one that everyone knows sucks. Maybe Anchorman 2

So Road House 2 is out then?

Mannequin 2: On the Move was so bad. Even Kristy Swanson couldn't save that waste of film.

I like it to a degree since I saw it as a little kid but Ghostbusters 2 has real a vitriolic stigma to it for many

Blair witch2
The fly2
Blade 2
Poltergeist 2

With some of the REALLY bad sequels the scripts don't even start out as said sequel. The studio execs get the script, have a few things changed from some script doctor and add the name of some franchise to it. Silent Night Deadly Night 5 is one exampled of that.

All of these are bad except Blade II. That was great! Blade III however.......awful.

Fletch Lives. Only because the original Fletch was so great. It wasn't particularly bad just forgettable. Only saw it once but it was a big letdown after the original.

The Sandlot 2

As the original Blues Brothers being one of my all time favorites. BB2000 was just an awful piece of shit! The jam at the end almost made it watchable. With that being said, BB2000 is the worst sequel ever.

The Prophecy 4

the non Walken edition

Yeah, there are bad sequels, awful sequels, and unwatchable sequels all in this thread, but Highlander 2 is in a class by itself. It had no redeeming values; it failed on every aspect; it is universally despised; and it was made with a straight face, as if they thought it was going to be amazingly good. It is the complete bad sequel to the level that it retroactively made the original worse.

The only other I can think of that was close is A Summer Story, the sequel to A Christmas Story, starring Macaulay Culkin's little brother. I turned it off partway through.