Sure! But rather than just telling you what I use, I want to emphasize there's not really a "this is definitively the best" or "this is definitively the worst" answer here. It's more about what you're trying to accomplish. AdGuard helpfully maintains a list of known DNS providers: Known DNS Providers | AdGuard DNS Knowledge Base, and their tabulation includes features of the different DNS providers (e.g., filtering, non-filtering, family, and so on). So, you might take a look at that and see what suits your fancy. I'm currently using Cloudflare and Google for my upstream servers and Quad9 and ControlD for fallback servers.
The only thing I'd say you should really consider is opting for some form of encrypted DNS, such as DNS over HTTPS (DoH) or DNS over TLS (DoT). There are pros and cons with respect to which type; some people get very passionate about this as well, but as with many things, the answer often sorta' ends up being "it depends". I favor DoH. If you want to read more, this is a good place to start: https://www.cloudflare.com/learning/dns/dns-over-tls/.
So, adding that all up: I use DoH servers from Cloudflare and Google for my upstream servers and DoH servers from Quad9 and ControlD for my fallback servers (and these are all listed in the URL I included above from AdGuard).
I hope this helps!