Glory, probably followed by The New World or The Mission or The Lost City of Z or Catch Me If You Can or The Pianist. I guess I also could never remove Schindler's List from my collection by any means.
PS: I'm seeing so many good ones I even own but forgot on here that are so good: Ed Wood, Bridge of Spies, Apollo 13, Joyeux Noel, Laurence of Arabia....some great suggestions. Citizen Kane is a film-buff essential too and inspired by the life of William Randolph Hearst. The Aviator and Beautiful Mind are very good; Rules Don't Apply may or may not be based entirely on a true story but that is also, like The Aviator, about Howard Hughes.