quad9 seems fine. Glad there are a bunch of alternatives though. We should never stop practicing decentralization in the net.

Be cautious with Quad9; their main address (9.9.9.9) has a "malware" blacklist that has misfired several times already: twice for a private torrent tracker, once for gist.github.com, issue was resolved within minutes to hours. They have a non-filtered address (9.9.9.10), but it doesn't do DNSSEC verification. IMO they're too unreliable to be worth the hassle.