Disclaimer, I work for Hotspot Shield and write for the blog!
Why is HSS the best free VPN? Because the company has handed over ZERO user data to governments since inception. The companies mission is literally to provide internet freedom.
Protecting your privacy is one of the most important factors for deciding which VPN is "the best".
All VPNs have small variances in product features, but one thing that is critical (for me at least) is if my personal information is actually safe.