I'm keen to hear from anyone who's registered a .rs domain and is happy to recommend the registrar they used.
The Serbian National Internet Domain Registry website has a list of accredited registrars but most provide their services exclusively in Serbian.
The only one I have found that provides a service in English is Burina.NET
Low prices are also preferrable.
No recommendation other than to say in my experience the best ones are the ones you have the least hassle of transferring out of.
whois is best way to find what others are using.
Which picks up with just short search (without personal knowledge of) istanco, ninet
I was tempted to mention hassle-free transfering. What would really be cool is a wildcard whois search api.