Mullvad’s DNS. It’s available for non-subscribers as well, and their privacy policy explicitly claims they do not log DNS requests in any way. https://mullvad.net/en/help/no-logging-data-policy/
They support both DoT and DoH, and also have various servers for blocking ads, trackers etc (if you wish to use them): https://github.com/mullvad/dns-blocklists
Well, if you’re using Mullvad’s malware/ad filters etc there’s really no need for a PiHole in the first place (unless you’re doing some funky custom filtering).