His Girl Friday
This is a must-see romantic comedy. Carey Grant and Rosalind Russell tell the story of a newspaper editor and reporter who work on a murder case and (of course) fall in love. It was preserved in the Library of Congress for its "cultural, historical, and aesthetic significance."
All the President's Men
This classic Robert Redford and Dustin Hoffman film tells the story of the infamous Watergate scandal. President Nixon's term was slated with various other scandals, but thanks to reports Carl Bernstein and Bob Woodward, Watergate takes the lead. The film was preserved in the Library of Congress in 2010.