In general, the order of the RRs as returned by the client resolver is undefined. Certain RR types allow for the specification of a precedence or priority - especially the MX and the SRV resource types.
If your implementation does not use one of these (and you asked for NS RRs which are unordered), you should consider all RR responses to be unordered as a conservative assumption.
This being said, most DNS server implementations would serve the RRs in the order you define them in the zone file unless you enable DNS-roundrobin. Caching resolvers may have roundrobin enabled but typically won't.
Client caches for IPv4 typically do not reorder all that much as well. With IPv6 things look different, though. RFC3484 Default Address Selection for IPv6 defines reordering in section 6 which is implemented at least in the Microsoft DNS Client.
What NS RR a DNS server recursing through to your zone is going to use in the end, is very much up to its implementation. Keep in mind that a (caching) DNS server would not rely on the operating system's implementation for resolving DNS queries but come with an own, likely including the ability to query multiple servers simultaneously and pick the fastest responding one for subsequent requests.