All Dogs go to Heaven 2 (Because fuck your childhood memories)
Apparently we didn't see any trailers for this movie, because we were shocked that it was a musical (and a really cheesy one at that) My dad kept leaving me in the theater for smoke breaks to suffer by myself. And they don't even mention Anne Marie in it. She was Itchy's owner, FFS!
The Secret of NIMH 2: Timmy to the Rescue (Also a lesson in re-writing NIMH's history)
I actually got this as a Christmas present, and my whole family sat down to watch it together on Christmas Eve because the original was a favorite in our house. It was so laughably bad. It also threw out pretty much any continuity from the original film. (Remember that scary Rat Brudus? The one that chases Mrs. Brisby around the rose bush? Yeah he's a goofy dumbass in this one.)
Land Before Time 2-20 (There has to be at least 20 of these now. They're like the film version of the Duggars)
Admittedly, I probably made it through 5 of these Land Before Time sequels before I realized I was just kidding myself and that they were horrible. My husband and I recently bought the original film on DVD for our son, and they had had some of the sequel songs as a sing-a-long for a special feature. We tried watching one and had to turn it off immediately because the opening lyrics were: "As an egg you were a beauty, but now you're a real cutie." I'm embarrassed just telling you about this.
Honorable Mention: Fern Gully 2: The Magical Rescue (I'm assuming this will also be the plot of Avatar 2)
The first Fern Gully already had singing, but there is one detail in this film that drove me nuts despite being all of eleven when this film came out. Batty. Remember Batty? At the end of the first film, he's small enough that Zak can hold him in his hand. Well, in this movie, he's big enough to fly away holding a kid. A human child, not a fairy one. Did he take some bat steroids in between movies?