While there are many options available, we recommend Quad9 and Cloudflare DNS in that order.
If you're running a cPanel+WHM server, then you can make the changes to /etc/resolv.conf through WHM > Resolver Configuration.
(Managed Server client? We manage this configuration for you - please contact us for more information!)
Quad9 secure-mode nameservers
9.9.9.9
149.112.112.112
Cloudflare public nameservers
1.1.1.1
1.0.0.1
Please note that any you pick-up via DHCP should be considered untrusted. Every server should have known-good resolvers configured.
As a backup to the above 2 options, you can also use Google DNS - however please be mindful of your data and how depended on DNS is.
Google public nameservers
8.8.8.8
8.8.4.4