Superman: The Movie - It birthed the genre and features an iconic performance from Christopher Reeve and John Williams' score.
Batman (1989) - Gave more legitimacy and was generational touchstone in the '80s and '90s, with also a historic performance from Jack Nicholson, and its Oscar winning production design, not to mention Elfman's score.
Spider-Man (2002) - More than any film kickstarted the modern superhero movie craze in the 21st century, has historical significance as a definitive piece of escapism in the shadow of 9/11.
Spider-Man 2 (2004) - Better film than Spider-Man and one of the most standalone "pristine" superhero movies with terrific storytelling, that kind of defines what folks imagine a "superhero movie" is.
Batman Begins (2005) - Created the modern "reboot" craze in Hollywood, turned master auteur Christopher Nolan into a household name, leading to the cache to make more "respectable" movies like Inception and Interstellar.
The Dark Knight (2008) - A masterpiece, one of the best films of the decade that turns the post-9/11 Bush years into paranoid, uncomfortable allegory. Plus, Heath Ledger's wonderful performance.
Iron Man (2008) - Began the shared universe adventure of Marvel Studios that dominated Hollywood cinema for at least 15-20 years onward.
The Avengers (2012) - Perfected the shared universe formula that all of Hollywood would chase onward.
*The Dark Knight Rises - Just by association and apparently predicting the ascension of real-life American super villain, Donald Trump.