Diagnostic symptoms of depression include: Depressed mood, Sleep disturbance, Loss of Interest (anhedonia), Guilt or feelings of worthlessness, Energy loss and fatigue, Concentration problems, Appetite/weight changes, psychomotor retardation or agitation and Suicidal ideation.
I think the worse symptom of depression is suicidal ideation because if eventually this patient carrys out this ideations ir may lead to death.