I think it would depend on your interests of course. There are many reasons why one might do a fellowship in x or y and there are many reasons not to. Depends on what your career goals are, how much you like training, where you do the fellowship etc etc. See my fellowships thread, but my ranking would be something like this:
1. Pain Medicine
2. Child and Adolescent Psychiatry
3. Behavioral Neurology/Neuropsychiatry
4=. Addiction Psychiatry/Addiction Medicine
4=. Forensic Psychiatry
6. Occupational and Environmental Medicine
8. Clinical Informatics
9. Sleep Medicine
10. Neuromodulation/Interventional Psychiatry
11. Hospice and Palliative Medicine
12. Headache Medicine
13. Geriatric Psychiatry
14. Psychosomatic Medicine
15. Public Psychiatry
This is based on skills learned, $$$ potential, and demand.
You absolutely do not need do a psychosomatic fellowship except if you wish to work at certain academic centers in the NE or wish to be a psychosomatic fellowship director. Bizarrely I was offered the position of PSM fellowship director at two reasonably regarded institutions despite not actually being eligible lol (ACGME requires the fellowship director to be board certified in that specialty, which of course makes sense). I could see how doing a PSM fellowship at a major quaternery center where you see a ton of zebras and get exposure to neuropsych, transplant, psychonocology, HIV psychiatry, women's mental health/perinatal, cardiac patients etc and get mentorship from leaders in the field might be pretty neat, especially if you did not get such exposure in general residency, but your average fellowship probably doesn't have much to offer.
A good geriatrics fellowship will give you strong training in serving this population and dementia in general, but most general psychiatrists work with older adults and no one cares about them. Also its all medicare. I was told by one place "we don't want you doing geriatrics because we don't want to attract more medicare pts", another place incentivized privately insured patients, and my friend was told at one place "you're worth less because of your geriatric training."
Headache medicine has no money in it, except maybe for botox. which is why they are open to psychiatrists.
palliative medicine no money in it, likely to decrease your income vs. general psych.
neuromodulation/interventional psych - very few jobs for non-research people. You could of course set up your own clinic, but would need referrals or collaboration and invest in equipment (e.g TMS chairs). but much of this can be learned in residency or out in your own practice. can be potentially lucrative if you are in an area where you can corner the market and have the business savvy and start up costs to invest appropriately.
sleep medicine is in decline because of declining reimbursements over the past decade. though you would learn a ton. doing expert witness sleep medicine work potentially v lucrative. may have difficulty getting jobs as a psychiatrist if they are looking for a pulm person or a neurologist.
clinical informatics is for the more technologically inclined. the market is being cornered by quite a few people but you can potentially get work consulting with tech companies, EMR companies, creating your own innovations etc. not necessary to do the fellowship for any of this of course.
OEM is gonna be big for psychiatry in the future as psychological injury, and psychiatric related disability including somatoform disorder (electromagnetic sensitivity, multiple chemical sensitivity etc) are the major causes of workforce morbidity - its no longer lung or skin disease. occupational/organizational consulting, doing certain kinds of disability evals and fitness for duty evals can also be lucrative. you would get training in general occupational/environmental medicine issues too which is the crux of what is currently offered
A good forensics fellowship will give you exposure and training in a wide range of criminal and civil competencies/evaluations and improve your overall acumen as a clinical psychiatrist. However it is absolutely not necessary to have a forensic fellowship in order to be an expert witness and the overwhelming majority of civil expert witness work in psychiatry is done by non-forensic trained people (BTW forensic fellowships have been around since at least 1969). Now often these people do pretty awful work, and are plaintiff's ****** (defense experts often tend to be very senior psychiatrists, again mostly not forensics trained but more likely to be), but they exist anyway. It's not necessary of course to be forensics trained to work in state hospitals, corrections, or community forensics either.
Addiction Medicine and Psychiatry is a growth area, especially with our iatrogenic opioid epidemic and the 21st century CURES Act. I am seeing more and more non-academic jobs demanding or strongly preferring addictions boards. Given that most programs provide substandard training in addiction psychiatry, it can definitely be worth doing an addiction fellowship. Potential to do MRO work interpreting urine drug test results, and lots of consulting/expert witness work relates to substance use issues. You can always sell out and open a bougie addictions practice or residential treatment centers. Though high-end residential is a saturated market and these places are struggling to fill with cash-paying or private insurance paying pts.
Behavioral neurology/neuropsychiatry - you will learn a ton in a good fellowship. clinical work is not usually well paying but can be. typically more academically oriented. forensic work can be highly lucrative especially related to TBI.
child - worth bearing in mind that vast swathes of the country general psychiatrists provide treatment to children (not usually great care) such is the need. in some cases with consultation with a general psychiatrist. I was recently asked to provide coverage for adolescents in a saturated part of the country, and declined. most people do not receive adequate child psych training during general residency (canada, UK, australia, NZ, and Ireland all require 6 months of child psych training).