This is a bit of an easy choice, but it’s no secret that Avengers: Age of Ultron is the least popular of the Avengers films. Many attribute this to the fact the film acts as a set-up for the next phase of movies. This leads criticism of Age of Ultron to focus purely on that area of the film, leaving out Ultron.
Ultron is one of Marvel’s most famous villains and if used correctly can be a terrifying enemy for the heroes to face. You can see this in the first trailer for the film and in his debut scene. (Where he references pinocchio.) However after this Ultron becomes a quippy and unthreatening character.
Marvel really dropped the ball with Ultron and it’s a shame he never reached the heights of his comic-book counterpart.