What's The Most Secure Windows Firewall?

The reason I ask is, Google isn't much help in this most important security matter. What do I recommend to my computer repair business customers who trust my judgement? I don't like to tell a customer that there is no perfect firewall/anti-virus/anti-spyware/anti-rootkit program, even though it's true, as any who has ever read the Phrack e-zine or 2600 magazine knows.