Film classics...is Casablanca all that?

I don't have an explanation for this but up until a week ago, I had never seen the film "Casablanca." I also watched "To Kill a Mocking Bird" for the first time a few months ago.

I was prepared somewhat to be let down by "Casabanca" and was surprised by how much I liked this film. Found myself thinking about it the next day off and on.

On the other hand, I was disappointed by "To Kill a Mocking Bird." The story seemed contrived to me and while Gregory Peck was tall, dark and handsome, the other acting in the film stank. I just don't think it holds up as a "best picture" over time. This seems to be the fate for this year's best film, "Crash." Not my best film...Hollywood's best film. My best film from last year is "Brokeback Mountain."

So, any recommendations for another classic film to watch? I've never seen "Mr. Smith Goes to Washington."