I'd say the worst mental illness to have is the untreated illness. Treatment might include medication, lifestyle changes, talk therapy, other therapy, recognition and acceptance of the condition, emotional support of self and from others -- but if it's a real illness, (and mental illnesses are real illness), then it deserves some kind of treatment. Having dealt with someone who had a serious, untreated mental illness, I'm going to say the worst mental illness is the untreated mental illness.