The Tor relays guide in trac makes that recommendation.
https://trac.torproject.org/projects/tor/wiki/TorRelayGuide#DNSonExitRelays
the guide has unbound examples but I tried to make clear that it is not the only option:
There are multiple options for DNS server software, unbound has become a popular one but feel free to use any other you are comfortable with. When choosing your DNS resolver software try to ensure it supports DNSSEC validation and QNAME minimisation ( RFC7816)
other popular DNS software like BIND didn't have RFC7816 support for long and I don't know if BIND supports RFC7706 (root zone on loopback) which is also nice to have but not as important as RFC7816.
in anyway prio 1 should be reliability, if it fails 100% of queries, it does not matter what kind of software is used or what kind of protocol features are supported and enabled.