The answer to this question is more difficult than it seems, as there are many pieces of research that contradict each other, even between such well respected groups as the National Alliance for Mental Illness; The World Health Organization; and The Mayo Clinic. What do these groups agree upon?
Let’s look at this from the opposite end first. As far as most difficult, which seems to be the more easily established question: Schizophrenia and other Psychotic Disorders, OCD, PTSD, Bipolar Disorder, Borderline Personality Disorder, Antisocial Personality Disorder, Pedophilia, and many of Somatoform Disorders have poor overall outcomes, either through the nature of the illness itself, its biochemistry, or the proper therapy compliance rate of the individual who have it. Most of these are known as Chronic Persistent MH disorders and tend to be very difficult to treat well in the overall population. Add a substance abuse problem to these MH diagnoses, and the success rate fall even further.
However, assuming that the patient is compliant with the recommended best course of therapy, some of the anxiety disorders, such as Specific Phobias; one of the mood disorders, Dysthymia (Chronic Depressive Disorder) and some childhood disorders such as Attention Deficit Disorder can be successfully treated and have excellent outcomes. But, some of these disorders are often (such as ADHD) not treated and complied with best practice treatment guidelines, and the outcomes can be very poor, again, such as in ADHD. So, outcomes can be contradictory.
Interestingly, one of the best success outcomes is in the area of sleep disorders, and by using a combination of good sleep hygiene and habits, diet and medication and even in some cases maxillofacial surgery, more than 80% of patients experience relief from a primary sleep issue, insomnia.
However, all research agrees: The best way to treat an MH disorder is prevention, and outcomes for prevention are 100% if the clinician successfully averted the problem. A best example is the use of psychological first aid in the treatment of potential PTSD related trauma where the intervention prior to the onset of symptoms can completely avert the disorder from forming.
I hope this helps,