I've used
http://www.CloudFlare.com which seems to be used by more people but was more of a good CDN without much of anything being blocked out in terms of security as I tested the free and the $20/month Pro plan.
Currently using
http://www.Incapsula.com free plan which is blocking more malicious shit out than the $20/month Pro plan from CloudFlare. Haven't tried a paid option here yet though and not sure I will as the free plan combined with a hardened server is working well. Currently have (444) domains running through them.
So if just looking for a popular CDN go CloudFlare.
If looking for a popular CDN with better security built in I'd go Incapsula.
