Do you mean the best registrar?
If you don't see price as a problem, and need quality, try Godaddy. In Godaddy you can buy domains, profesional emails, hosting, website builder, US phone number, and even you can hire a profesional web developer and monthly maintenance for your website. All with the best quality, but the price is very high.
If you need only domains with cheap price, try Namecheap, Dynadot, Porkbun, Epik. For me, Dynadot has the best price with the DNS service easy to handle my self. I use Dynadot domains attaching for Godaddy Website, Blogger, Godaddy Email with no problem.
If you want to buy aftermarket domains with rather high price, with a chance to get a loan against domains, try to use Epik. Only Epik has this feature.