What is the best liability insurance for an nurse practitioner?

The one that gives the best quality product at the best price.

Quality is a function of policy service, claims service and, I believe, risk management analysis and education.

Before shopping, understand the difference between claims made and occurrence policies. Claims made will be cheaper in your first year, but you need to understand tail coverage. The costs are about equivalent in the long run when you consider the tail coverage.

There are very few carriers out there. Talk to your state professional association about what carriers operate in your state. They may also have a group discount for members, as may some of the national NP associations.

In some setting, you can be added onto the group's policy as a named insured, but you may choose to carry your own.

For a more personal answer, I've had NSO and now I have CMF.