Sound

I still think sound is the most important component of language. From what we've learned about two of the three (syntax and phonetics) I think phonetics makes the most difference in how language is interpreted. Syntax can be learned and vocabulary can be memorized, but new sound patterns are still very difficult to learn and produce over a short period of time. For example, Americans who live in Britain usually can pick up on the different words for things and the different sentence structure very quickly, but it usually takes years for them to lose their American accent.
Besides being the most important to understanding language, sound is what makes it unique. Without accents, everyone who spoke a certain language would sound the same and it would be no fun.