X-Men earns this label because it was certainly the comic book movie that changed how the genre was perceived from that point on, and helped launch the current superhero movie Golden Age we live in.
20 years of X-Men
Naturally, the first X-Men movie set the stage for what was to come. Marvel Studios head Kevin Feige was an associate producer on that movie and produced every movie made with the Marvel branding until the MCU began in 2008. But on a narrative level, X-Men set up the central tenets of its universe very well.