I feel Casablanca is a timeless story for a couple of reasons. The first is the clever dialogue in this movie. The fact that several phrases from this movie have become part of our language clearly shows the power of the dialogue; for example "Here's lookin' at you kid." Rick gives this toast several times during the movie. Ask most people (of a certain age) and they will know that this phrase was said by Humphrey Bogart (at least) and many would know its origination.
One of my favorite lines is in a conversation between Rick and Ilsa. They were reminiscing about the day they met. Ilsa asks Rick if he remembers the day. Rick responds "Not an easy day to forget; I remember every detail. The Germans wore gray. You wore blue."
There are so many examples of clever dialogue, but I won't go over every one. However, another favorite of mine would be the following. When asked "What is your nationality?", Rick responds "Drunkard."
The screenplay for Casablanca was not adapted from a book, and several writers are credited (and uncredited) with creating the screenplay. I think this makes the achievement even more surprising.
Ironically, the most well-known "quote" from the movie is in fact a misquote. "Play it again, Sam" is not in the movie.
The second reason this movie is a classic is the characters and the choice of actors to portray those characters. Nobody else could have played Rick. No other actress in the world could have played Ilsa. This is one of those magical moments where the right actors combined with the perfect screenplay created an unforgettable movie.
And we must give kudos to the supporting cast as well. Peter Lorre as Ugarte, Claude Rains as Captain Renault... the list goes on and on.
If I have to choose one character it would be Rick Blaine. Bogart's portrayal of Rick gave the character an emotional depth found in few fictional characters. Bogart's eyes broadcast a thousand emotions, yet his face remains inscrutable.
Casablanca will always remain on my list of classic movies.