As a company, we own a lot of domains, so we like to spread the load. We use lots of registrars. Of the major ones, I'd say Namecheap and Gandi are the best. We also use DNSimple who are good but have since moved to "I Want My Name" for the newest ones since they don't require you to upgrade your account the more domains you buy (they price their services into the domain cost instead, which works better for us).

All that said, I'd love to have a registrar that just sent us an annual bill for all of our domains (even if it's in advance) rather than billing us separately for every one. It's an accounting headache with all the expenses. I believe GoDaddy does that, but.. no thanks :)