Which is the best way to diagnose an active COVID-19 infection, the molecular PCR test (nasopharyngeal swab) or the serology test (blood draw)?

A molecular test (PCR) performed on a nasopharyngeal swab is the current gold standard for diagnosis of an active COVID infection. 

A positive serology test cannot distinguish between a current or past COVID infection. A negative serology test does not exclude a prior COVID-19 infection. It takes time for an immune response to develop after a COVID-19 infection. So for example, if you were sick and your symptoms resolved 10 days ago, a repeat serology in 1-2 weeks may be helpful.

Source:
https://stanfordhealthcare.org/health-care-professionals/covid-19-test/serology-testing-faqs.html