“X-Men” laid the foundation for all the great Superhero flicks to come
I think the X-Men films have been so popular because the X-Men dare to be different. The concept of the X-Men strays far from superhero conventions. If you approach the X-Men films thinking you're getting something akin to Superman, Spider-Man or Batman, forget it.
The unique concept of the X-Men is that humanity is starting to evolve to the next level and a small percentage of humans all over the globe are starting to manifest superhuman powers from the mutant "X" gene. Two mutant leaders, who are old friends, take totally contrasting positions: Charles Xavier starts a school for mutant youths in upstate New York. His attitude is positive and his goal benign. Eric Lensherr or Magneto, on the other hand, is fed up with the paranoia of non-mutants. He starts a "Brotherhood of Mutants" with an attitude of superiority. Unfortunately, power corrupts and absolute power corrupts absolutely.
You could say that Xavier takes the approach of Martin Luther King Jr. while Magneto goes the route of Malcolm X, an interesting comparison.
Although everybody has their favorite, I feel all three films in the original trilogy are of the same general quality: "X-Men" (2000), "X2: X-Men United" (2003) and "X-Men: The Last Stand" (2006).
"X-Men" naturally lays the foundation by introducing the principle characters and the plight of the mutants. Wolverine and Rogue are introduced and slowly develop an interesting big brother/little sister type relationship. They come into contact with both Magneto's Brotherhood and Xavier's school and nobly opt for the latter. Magneto's bunch includes Sabertooth, Toad and Mystique, while Xavier's team includes Cyclops, Jean Grey and Storm, as well as adolescents Iceman and Pyro. Both groups of mutants meet and tussle at Liberty Island in New York Harbor, which is the weakest part of the film, but not bad.
When "X-Men" debuted in 2000 it was easily the best comic-to-film adaption since "Superman" (1978) and "Superman II" (1980). It was the precursor to all the great superhero films that came out in the 2000s and has only lost some of its allure due to the quality of many of those films.
The movie runs 1 hour, 44 minutes with several interesting deleted scenes available.