My experience
Quad9 has the best quality blocklist which they receive from their sponsors. IBM is a big player (mainframe and network switches supplier). Their other partner Global Cyber Alliance also has a lot of security companies participating. Combined with open their source filters they probably have the best filters when I take only free DNS services into account. Quad9 does not has any AI-based filtering in place.
NextDNS offers a lot of security features related to the DNS service itself. Their AI-based filter and Google safe search makes them a good choice for people using Edge with Microsofts Smartscreen feature. Knowledgeable users can raise the bar by blocking newly registered domains, blocking all non-latin character TLD's, enabling some parental categories which they don't use to reduce malvertising (e.g. gambling, piracy, dating and porn).
I don't have any experience with CleanBrowsing, so can't provide feedback on that
I used to prefer Quad9, but after reading some articles about how successful AI-based DNS blocking is at Cisco and Cloudflare 1.1.1.2, I started to look at NextDNS. After all the initial horror stories of Google's MV3 killing adblocking as we it, I started to use NextDNS because of the unmatched level of control they provide for free and the option to add privacy based filters NextDNS, AdguardDNS and OISD).
I know that some people are quite convinced that malware blocking should be done at DNS and adblocking on the client or in the browser (which makes sense in theory), but I can say that I use Next DNS for nearly a year now and have not run into a situation that the NextDNS adblocking caused a website to break (but that is my practical experience only).