This is a landmark study – a meta-analysis by Dr. Keith Dobson that reported that cognitive therapy was even more effective than medication treatment or other psychological treatments for treating most depression. Medication treatment, however, when added to cognitive therapy with more severe depression, results in the best outcomes:
http://psycnet.apa.org/?fa=main.doiLanding&fuseaction=showUIDAbstract&uid=1989-30221-001