1. Akamai is by a huge margin the single biggest CDN in the world, they are the 800lb gorilla. Fastly and Cloudflare aren’t minor players by any means, but their volume is not in the same league.
2. CDNs and DDOS don’t have much to do with each other. Cloudflare mitigates DDOS by scaling up network capacity and using pretty advanced pattern detection to simply soak up the traffic. Cloudflare is really, really good at scaling.

Now on that last point, there will indeed come a time when simply using the engineering technique of “making things bigger” won’t work if the attacks become sophisticated enough, but at that point networking will have fully become geopolitical tools (more than they are now).