CBT is effective in mild to moderate depression along with psychotherapeutic techniques and coaching. It is also a useful adjunct to medication in significant to severe depression.
ECT is useful in severe depression that does not resolve with medication.
I believe the research shows that even in severe depression cases, Cognitive Behavioral Therapy can be powerfully and lastingly effective. It does depend on a very skilled therapist and a great bonding, connection with the client.
In most cases in Qx's therapy cases, those on medication (which he supported in his earlier years) tended to be much more difficult to help rewire their stinking thinking. They were much harder to connect with. It was as though the meds were a kind of buffer that prevented the client from facing life and relationships even in redemptive ways.