9. The Dark Knight Returns - Batman V Superman
Since you were a school kid, you've probably been hearing about it: Batman versus Superman, who would win? It's one of the favorite arguments of any geek. It's like "Could God create a rock so big he couldn't lift it" but nerd-flavored.
That entire argument got turned into a film. That's bad enough as it is, but the movie - which itself is blatant fan service - decided to double down. Not content with having a fanboy argument take the place of a plot, this film decided it needed to rip from the hotly contested title for best Batman comic ever, The Dark Knight Returns.
In that film, a geriatric Batman fights mutants, monsters, and eventually Superman. Since he's eighty thousand years old, he uses a mech suit to help him stand up. You see where we're going with this?
Yes, Batman versus Superman: Dawn of Justice (again, even the title is a winking fan service nod), decided it needed to include the mech suit Batman. Of course, the movie doesn't have a very good reason for it, as the fight ends with most of the mech suit destroyed and Superman beat by a Kryptonite spear.
The only reason for the mech suit fight was because the studio knew as soon as you included it, fans would gobble the entire film up. Almost the entire film is built to justify this one scene and it doesn't even do it well.
Just the worst.