"Wreck-It Ralph 2: Ralph Breaks The Internet". I didn't like how game-jumping was viewed as something negative in the first movie, both for its' title character and the film's villain, King Candy, but seen as something positive for Vanellope in the sequel. The fact that Wreck-It Ralph is chastised for his actions in the second movie, but Vanellope didn't catch any heat for her actions, ruined the movie for me.
"Jem And The Holograms" was also a big disappointment for me as well. I think several things wrecked it, most notably changing everything that made the cartoon so unique to turn the film into a generic band movie, and giving the movie a budget of 5 million dollars when other Hasbro properties turned into movies had budgets in the nine-figure range. $5 million couldn't even cover 10 minutes of special effects in a Transformers movie, so how could they make a decent Jem movie for that budget?
Also, I didn't like "Jack And Jill", either. I think Cinematic Excrement's review of the movie is funnier than the film itself:
For that matter, Cinematic Excrement also did an excellent take on "Jem And The Holograms" as well: