I am a licensed therapist. which accounting method is best for tax purposes?

For most self-employed people and small businesses, the cash method of accounting is the preferred method, as it is easier and simpler to implement than the accrual method.

Please read this IRS publication for more information.

which is best can vary from year to year but generally, most would use the cash basis.  income is taxable in the year received and expenses with some limitations are deductible in the year paid. with the accrual method income is recognized in the year the services are rendered and expenses, again with some limitations, are deductible in the year incurred. 

more info IRS PUB 538

https://www.irs.gov/pub/irs-pdf/p538.pdf