Here are some of my favourite films/scripts to use in studying screenwriting.
1) Miller's Crossing: the Coen's best ever. Gets absolutely everything right.
2) Some Like It Hot: Billy Wilder and I.A.L. Diamond at their best - though recent changes in the law in Belgium and select other countries have made the funniest scene of the movie outdated.
3) Die Hard: though written by committee, just about the ultimate 'traditional' Hollywood movie. One of the best to illustrate screenplay structure.
4) Million Dollar Baby: another very well executed traditional screenplay structure with a third act which goes completely in a different direction from what you'd expect.
5) Casablanca: just because. It's Casablanca, man!
6) Star Wars: not a great script but a very effective teaching tool for illustrating the Hero's Journey and sequence building.
7) Strangers On A Train: brilliant subtext from almost the very first scene. Hitchcock and Chandler hated each other but created movie magic.
8) Bullets Over Broadway: not my favourite Woody Allen movie (that would be Love And Death) but a very well done comedy with a lot of well-observed jokes about writers and actors.
9) Career Girls: A Mike Leigh film with flashbacks and a structuring event (a weekend spent together) rather than a clear 3-act structure. Off-beat and interesting and with great quirky character work.
10) On Connaît La Chanson/Same Old Song: French multiplot comedy with lip-synched French popular songs by New Waver pioneer Alain Resnais in his (much) later years. A very good use of multiple but connected plotlines and an excellent example of the effective use of theme in a script: each character in the movie lies to themselves (and to others as well, but primarily to themselves), yet at no time does this come across as forced or artificial.
There are many, many more out there, of course, but these 10 have all been used with great success in my courses. Watch, rewatch, study, learn and assimilate!