For 4 years I have been an advocate of using Namecheap for domain registrations. They have had the occasional technical problem, but overall I am extremely satisfied with their service, their interface, and above all knowing they are not going to switch off a domain just because of a single spam complaint.
At least one of Namecheap’s major competitors will switch off your domain and force you to pay a huge processing fee to switch it back on, even if the spam complaint was for an email that just listed your domain, or an affiliate link that went to one of your sites, sent by a 3rd party.
That can happen at any time, and Sod’s / Murphy’s / Finagle’s Law it will happen at the wrong time, such as a major product launch.
I have been told by friends that Moniker are also reliable in this regard, though I have never used them for normal domain registrations.
Some time ago during other support requests with Namecheap I asked about a possible affiliate program, as whilst it would never pay big on domains, it could offer a little more on things like SSL certificates and maybe hosting.