Spaceship: My go-to since they launched. Low prices, slick UI and great support.

Dynadot: My previous go-to. They have some features that Spaceship could use but also somewhat higher prices. You can't go wrong with either.

GoDaddy and Namecheap: The top two registrars by number of registrations. Sometimes you have to use them but Spaceship and Dynadot are better.

Unstoppable: Newcomer with great prices and some nice features.

Cloudflare: Solid prices and convenient if you use their other services.