NearlyFreeSpeech.NET is the registrar I use. They're a web hosting company, so domain registration is not their only service, but they have an excellent company philosophy and customer-friendly policy set.

I've been using them for about 10 years now, and they have been extremely reliable, zero-maintenance; and the web-interface is very easy to use and logically organised. (no javascript!)

Their prices aren't too bad either, since domain registration isn't their main priority.

Only downside is that they don't support all domain types, only the legacy GTLDs and a small list of new ones: https://www.nearlyfreespeech.net/services/domains