sneer

While just as anger inducing as a smirk, a sneer is a sign of absolute contempt. A sneer sends the message that the person sneering believes the target of their ire is so utterly pathetic that they want to laugh. It's an extreme expression and should be used sparingly.

If it's not a smile, use disdain or disgust instead. If the emotion presented is anger or hate, use scowl instead.