Unequivocally one of the most important movies in cinema history. I'm of the opinion that this is the movie that is imperative to watch at least once in your lifetime, as it teaches some incredibly influential lessons that are fabulously presented by actors like Robin Williams, rest his soul, Matt Damon and Ben Affleck. This movie is in the genre that are the most important for one to watch, because they are movies about people. Relationships, insecurities, childhood trauma, but most of all - development.
Good Will Hunting shows the development of a character in no other way I've seen in movies before, with Will starting out as a typical troublemaker always looking to push your buttons and sniff out what makes you tick, and the reason for this becomes apparent as you watch the movie. But by the end, he's a transformed man, a mirrored version of the Will we are introduced to at the start of the movie.
This film is also unique in the way it teaches its lessons - through Will's therapy sessions throughout the movie. Normally you'd have a conflict that is resolved by the end of the movie to preach its message to the viewers, but instead this movie intends to tell you what it wants to through Will's therapist, almost like what he has to say isn't just for Will, but for you too.
It's an undisputed masterpiece in my book.