Depending on the global top level domain extension (gTLD) that you want to register. It maybe better to choose a small domain registrar versus a large domain registrar. If you want to register a lot of the new gTLDs than you may want to go with the registrar that offers the selection of new gTLDs that you desire. If you want to just register a .com domain than any registrar that has a good reputation for support may fit your needs.
You should look at the domain registrars that you are considering and see what their prices and selection of products are. If their website is up to date than you should be for the most part good to go. Whatever you shouldn’t do is choose a totally unknown registrar that has a really old looking site. The support system in place may not be very good if their website is not easy to navigate.