Father-in-law is a dentist. Beware of the overhead, especially the start up costs. Unless you're independently wealthy, it is ungodly expensive to start a new dental office and you probably won't be able to afford it. If you do, you'll have to take out a loan (unless you're already rich), and then there's a decent supply of dentists, so you'll be slow building patients and likely go under.
For medicine, there are only a few fields that are still conducive to starting your own private practice, for the same reasons. Dermatology is pretty good, but is crazy competitive to get into. You won't get in most likely. If you do, they pay, overhead, and hours is pretty darn good. The other choice would be Child Psychiatry (or maybe adult psych, but child is better). Child Psychiatry benefits from absurdly low overhead (probably the lowest in medicine), low startup costs (office, cheap furniture, laptop, maybe a website), and an insane shortage of doctors. Every child psychiatrist I know only takes cash (no insurance), $250-350/hr, has more patients than they want (wait times are around 6 months for an appointment in most areas, and that's if you have the cash), and overhead runs in the 10-20% range, compared to 60% for most medical/dental offices. It's a great field and this is a huge reason why I chose it and why I'm here on this part of SDN.
