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.
Sucuri - Website Protection, Malware Removal, and Blacklist Prevention
cdnjs - An open source community driven JS/CSS CDN.
Amazon CloudFront - Amazon CloudFront is a content delivery web service.
CloudFlare - Cloudflare is a global network designed to make everything you connect to the Internet secure, private, fast, and reliable.
Statically - The free optimization & CDN for images, CSS, JavaScript, and open source.
KeyCDN - KeyCDN is a high-performance Content Delivery Network (CDN). Lowest price globally at $0.04/GB with HTTP/2 Support and free Origin Shield.