Google free DNS server 188.8.131.52 and 184.108.40.206 has been used by me for many years, since it is decent fast and also easy to remember. But now Cloudflare just announced their DNS server 220.127.116.11 and 18.104.22.168 on April 1st 2018.
Here is their comparison diagram with other free public DNS server providers.
Not only fastest, Cloudflare pledged that it will not write your ip to the disk and not keep your tracking data long, it will purge all logs within 24 hours. How cool is this privacy focus feature? Definitely, I will give it more try later if there is any need to use a public DNS server.
List of free public DNS servers:
DNS Provider Primary DNS Server Secondary DNS Server
Cloudflare 22.214.171.124 126.96.36.199
Google 188.8.131.52 184.108.40.206
OpenDNS Home 220.127.116.11 18.104.22.168