Important is so subjective. Find out what your kids are interested in and go from there! IMDB is great for searching movies.
ETA: I read your post more closely and see that you are talking about movies that are classics? Movies that have stood the test of time, perhaps? Google for the top 100 films of our time. Now, not everyone is going to enjoy those films or think they are important. Each person has to decide what they feel has meaning. I'm a cinophile but many of the movies I enjoy are not movies I feel my daughters or stepchildren will like or consider "important."
My eldest daughter, 16, enjoys manga, anime and Japanese horror films. My eldest stepd, 23, won't watch anything remotely "sad." She enjoys campy horror films and action flicks and some comedy. My eldest steps, newly 21, enjoys indies, arthouse films, classics, blockbusters and fratboy comedies. My youngest steps, 19, likes classics, musicals, blockbusters, b&w films and romcoms. My youngest daughter, 7, likes musicals, anime, Disney and "classic" family films.
Sometimes, however, I am surprised by what my children like. Recently, I watched the entire Thin Man collection and my youngest d became fascinated by the dog Asta (and the "fast" talking, as she called the banter). When someone in her future speaks of the dog Asta, she can understand the reference. Is that what you are looking for? Films that may be referenced in the future and you want your children to "get" the reference?