I currently host domains with the following registrars:

• Cloudflare Registrar - At-cost domains, but limited number of TLDs
• Porkbun - Cheapest one for most nTLDs
• INWX - Cheapest one for my .de and .eu domains
• OVH - Cheapest one for one .se domain I own

I only consider registrars, when they meet these requirements:

• Support for DNSSEC (Tool to test DNSSEC)

The cheapest registrar can often be found via tld-list.com.