For a business, MarkMonitor. They provide brand protection, super-locking domains (if another registrar tries to change root server records, they will change it back and contact that other registrar) and many other things. If someone managed to get one of your domains, their team can give your lawyers everything they need to quickly get it back. They have real customer support managers and respond quickly to support requests. You can get a real person on the phone instantly. Their CSM's can assist with migrating domains from all other registrars and know all the issues with each. Some countries have unique and odd restrictions.
To be frugal I check registration prices using tld-list[0] before purchasing a new domain.
Generally, though, I stick with Porkbun[1]. Their prices are decent and I prefer their site over Namecheap, 101domain, and a few others that I've used.
1&1[0]. The UI is super and focused on your productivity and not on selling you things when you're trying to edit your domain settings. I've used them exclusively for 20 years. Also, they have great prices and intro offers if you are buying a name for a new project.
