Is there one best test to diagnose appendicitis?

CAT scan: Good history and physical exam labs and if needed a cat scan to confirm diagnosis.

Clinically/CT scan: Often, the diagnosis is a clinical one, made by an astute physician based upon history and a good physical exam. Most doctors will now confirm their impression with a ct scan. There are other ancillary tests which may support the diagnosis but which are not always necessary (white blood cell count, sed rate, crp, etc.).