Countries operate in multiple countries and every country and state that a company is domiciled in has a tax rates prescribed by law.
There is no one size fits all tax rate that is best when valuing a company using the DCF valuation approach. Multiple tax rates apply to a company. Federal /state statutory tax rates, Effective tax rates, marginal tax rates, etc.
This page addresses the question: What tax rate would you use if your cash flows are international and have different tax rates in different jurisdictions?