(Warning: Spoilers for those who have not seen X-Men 3: Last Stand)
X-Men and X-Men 2 were commercially and critically successful, while X-Men 3 was just commercial successful. The movie was criticized for it’s plot and death of some major characters like Charles Xavier(even though at the end of movie, it is revealed that Charles Xavier particles have collaborated again and he is alive). 20th Century Fox saw no future in continuing the story and instead of making sequels, they started making prequels. First one was X-Men Origins: Wolverine released in 2009. It was critically panned for its special effects, acting, storyline, characters and direction. Due to it’s commercial success, 20th Century Fox still saw potential in X-Men series and released X-Men: First Class in 2011. Unlike X-Men Origins: Wolverine, where the protagonist Wolverine was played by same actor as original i.e., Hugh Jackman, here the protagonist characters, Magneto and Charles Xavier were played by different actor since it was about young age of these two characters. Now you would be surprised when you read Magneto and protagonist in same sentence, for he was the main antagonist of X-Men trilogy, but this movie was about the time when Charles Xavier and Magneto were friends and had not turn against each other.
While the old Magneto and Professor Xavier were played by Sir Ian McKellen and Patrick Stewart respectively, the younger ones were played by Michael Fassbender and James McAvoy. The movie was set in 1960s during Cuban Missile Crisis and was both commercially and critically successful. This gave confidence to 20th Century Fox to make sequel instead of prequel only. 20th Century Fox thus made Wolverine in 2013 with a planning to make a movie on Uncanny X-Men comic book storyline “Days of Future Past”. That planned movie was released as “Days of Future Past” in 2014.
In the movie, Days of Future Past, Charles Xavier is a full fledged character as before. The movie has Kitty Pryde having power to send Wolverine back to 1973 as enemy of mutants called Sentinels are killing mutants in present with the help of the DNA of Mystique they possessed in 1973 and Wolverine will not let this happen.
Plot wise this had most layers than any other X-Men movie. Even more layers than X-Men : First Class. This movie was biggest cinematic moment for X-Men fans. Not only it was based on most well known story of X-Men but it was directed by Bryan Singer, the guy who directed X-Men and X:Men 2, which were till then most loved X-Men movies.
It was loved for it’s plot, action , acting and direction. However personally I liked X-Men Last Stand more entertaining and X-Men First Class better. But that ‘s just me.