Many cite The Gold Rush as the definitive Charlie Chaplin film. Many others go with City Lights, which is the one I would pick, too. It is the culmination of all Chaplin's Little Tramp films, masterfully juxtaposing humor and sorrow to obtain a wonderful sort of bittersweetness that few, if anyone else, can achieve. The boxing scene is a masterpiece of comedy, while the scene where the tramp buys a flower from a blind girl is one of the most poignant moments on film. (Chaplin shot that scene 342 times before he got it the way he wanted it!)

It was 1931 when City Lights was made, and sound films had been produced for four years by that time. City Lights is often considered the last great silent film. Although it does have a soundtrack, sound is used only to illustrate how unnecessary it is and how meaningless it would be in the execution of his craft. Later, Chaplin would find a way to do wonders in the medium of sound also, but the Little Tramp character is above such pedantic forms of expression.

If you're not familiar with silent film -- if you're interested in it or if you're doubtful of the potential -- then there are a couple different films I would recommend you start with: The General is one. City Lights is another.