Is it one that you just use and works just fine? Or one that has proven to be reliable and responsible if they do a mistake and only want to satisfy you as a customer?
Cloudflare works really well and has a good UI. Namecheap also works well, but it takes more clicks to adjust DNS records.
+1 for Cloudflare.
That said, there are a number of folks rightfully concerned about the sheer mass of information Cliudflare has access to through their Content Delivery Network (their primary service). This raises potential privacy concerns, especially for self-hosters, who tend to prefer not to rely too heavily on any one large company. However, you don’t actually have to use their CDN service to make use of their minimally-priced Registrar functionality, and personally I really appreciate the services they offer. Their free tier is really impressive, and incredibly useful.
For some reason every registrars dns panel has its own weird restrictions, bugs and interface quirks. Pointing the nameservers to Cloudflare at least makes for a consistent experience.