A video of Audrey Hepburn from Breakfast at Tiffany's. As miscast as she was in that movie, it showed her at her most (incredibly) beautiful:
Truman Capote, who wrote the book, said that the part of Holly Golightly (an extroverted prostitute) called for a Marilyn Monroe type, which Hepburn was most definitely not.
Monroe famously sang "Happy Birthday Mr. President" to John F. Kennedy on May 29, 1962. But in fact Kennedy's favorite actress was Hepburn, and she sang "Happy Birthday, Dear Jack" to him exactly a year later, on what would turn out to be his final birthday. (The reason Hepburn's rendition isn't as famous as Monroe's is undoubtedly because it didn't feature the same subtext.)
Monroe, in her own dysfunctional way, was the archetype of a modern movie star: drug-addicted, irresponsible, and incapable of any stable relationships. Hepburn, who devoted the last fifteen years of her life to working for UNICEF, was pretty much the opposite.
Hard to blame Monroe for what she was, though. Read the "Family and Early Life" section of her bio on Wikipedia, upbringings don't come much more unstable than this: