The answer to 'best backflow preventer' is a reduced pressure zone (RPZ) backflow preventer. An RPZ backflow preventer is considered one of the most effective and reliable devices for preventing contaminated water from flowing back into a clean water supply. It uses a series of check valves and a relief valve to create a physical air gap, preventing any backflow from occurring.
RPZ backflow preventers are commonly used in high-risk applications, such as commercial and industrial facilities, where the potential for contamination is high. They are typically installed at the main water service entry point to a building, ensuring the entire water supply is protected. RPZ devices are required by many local plumbing codes and are inspected and tested regularly to ensure proper function.