CloudFlare since they sell domains at cost and have really good DNS infrastructure with some free protection features. If the TLD isn't supported by them for registration then I'd just use their nameservers.

Or Route53 if you're using AWS since that makes it easier to integrate with the rest of AWS and manage in IaC, and AWS also has robust network/DNS infrastructure.

(I would say GCP if using GCP/Google Workspace, too, but since they split domains off to Squarespace I really don't know what is happening over there anymore as far as domains go.)

So far those 3 have been more than sufficient for all of my domain needs.

If it is extremely critical, MarkMonitor.

Otherwise, Porkbun or Cloudflare Domains if you're ok using their DNS.

I suspect you mean register/renew:

Depends. If it's something really high priority (like main domain for a large corporation) I'd likely be paying CSC 4 digit sums per domain per year.

For stuff a tier below that I'd be looking at companies that are serious about security and happen to do domains as well e.g. Cloudflare, Amazon

Literally anywhere else.

Literally anywhere else, GoDaddy is utter trash and has been for many years. Namecheap is the one I use personally.

Porkbum or Gandi or name.com