In the right market, I bet opening a full-scale pain clinic would be most "profitable". No big million dollar machines required. Employ enough midlevels, churn through patients.
That said, as always, the practice environment in medicine is fluid. There was recently some big changes in pain outlook with nurses being able to deliver treatment independently, sooo who knows.
I guess, it really depends how far we are supposed to wander with your set parameters. The handful of psychiatrists that charge $500/hr obviously make a healthy profit. Their overhead is verrry small compared to other medical specialties.
However, taking into account the rarity of those providers (i.e. <1% of psychiatrists), is it even worth considering?