There are movies that are not made to be realistic or believable - sci-fy, horror, James Bond, Raiders of the Lost Arch, etc. I know going into those what to expect and I still find them entertaining. But when a movie is supposed to be believable and the characters behave in a way that no one would, that's what pisses me off.
The movie The Game which starred Michael Douglass and Sean Penn was one of the most ridiculous plots ever. And the ending... only in America could someone get rich making a movie like that.
I enjoyed The Game, thought it was a good movie as I do most of David Fincher's movies. RT, imdb, metacritic, all pretty positive about it. Yeah, there are plot holes, but entertaining overall.
A lot of the movies listed in this thread aren't like 5% in RT or under 3 in imdb, we're talking Gigli or Grown Ups 2 (7% RT.) bad. We're going for worst ever here. Really bad, not I didn't like this one myself, but utter hatred from everyone. Tanked at the box office, tanked with the critics, tanked in dvd/streaming ratings. And not so bad, it's good, like Sharknado.