Depends so much on the quality of training where one is, and what one is considered to be qualified FOR.
It's not like you are comparing apples with apples.
Psychiatrists are generally better trained when it comes to organic issues....they often THINK themselves better trained when it comes to internal dynamics...but (at least here) they are often using fairly outmoded theory as if it were gospel truth.
Psychologists are generally better trained when it comes to evidence based theories of mind and behaviour.
Social workers are generally better trained when it comes to systemic and sociological issues.
This is why multi-disciplinary teams are so useful.
That being said, there is huge individual variance, and it often comes down, a few years after graduation, to the kinds of further education one has done, and one's experience and reflective capacity.
If my friends ask for referrals, I am not guided by discipline, but what I know of an individual's quality of work, their suitedness to the individual in question etc.
I have several psychiatrist friends who have sought therapy from social workers, or, in one case, for a very specific CBT program, a mental health nurse, and in one case a psychologist.
I have a friend from where I last worked who is a social worker, but she is supervising 6 clinical psychologists, because they fought to have her supervise them because of the extent of her skills. (They also have to spend time with a psychologistsupervisor, but their therapy supervision is from her because she is outstandingly better than the psychologist seniors...though they are fine practitioners.)
Personally, I greatly miss having at least one psychiatrist on team where I am now. The more and different the perspectives, in my view, the better the service, and the more one learns.
By the way, if one were to poll most workers in the field, (in Oz) leaving out psychiatrists, psychiatrists would likely be seen as the least likely to provide good therapy.
I am often a lone voice in such discussions as maintaining that there ARE very good psychiatrists around who do great work. This is because I have met and worked with them.
There are also some remarkably fine psychiatrists researching and working in the fields I specialise in...complex trauma and attachment, and many of the books I gain most from are by psychiatrists.