As a field, psych is typically easiest. You really just need four walls, a couch/chair, and a notepad/laptop.
I'm solo inpatient rehab (PM&R--not psych/ drug rehab). Almost no overhead (just malpractice, which is pretty minimal, plus biller fees, professional memberships/licensing, etc). I could easily do SNF rehab that way, and plenty of docs do. Same goes for SNF IM work. My overhead is less than any solo psychiatrist because I don't pay rent.
Lots of FM docs still solo.
The problem is most of us are afraid of going into solo practice--I was! And like I said above, I have almost no overhead. But I learned nothing about practice management in med school/residency, and wasn't aware of how much more control I'd have over my life, how much more I'd make, and how much less I'd work, all be being in independent practice.