It's a cliche to choose as a favorite for a reason. It's that good. It's funny, it's exciting, Nazis get clowned on, plans fall apart while others come together, and it wraps up in less than 2 hours.
Some cliches *become* cliches because there's such a resonant level of truth to them.
But in the case of Casablanca, I think it's an instance where the legend became the fact. There's no overrating that movie because it deserves every good word it has coming to it, and THEN some.
