jsDelivr's answer:
jsDelivr is the only free CDN that aims to be production ready even for the most critical workloads. It has multiple failover systems for all of its components to ensure 100% uptime and uses multiple CDNs and datacenters for better performance.
Amazon CloudFront - Amazon CloudFront is a content delivery web service.
cdnjs - An open source community driven JS/CSS CDN.
CloudFlare - Cloudflare is a global network designed to make everything you connect to the Internet secure, private, fast, and reliable.
Netlify - Build, deploy and host your static site or app with a drag and drop interface and automatic delpoys from GitHub or Bitbucket
Statically - The free optimization & CDN for images, CSS, JavaScript, and open source.
Fastly - Real-time CDN