Ideally, both. In the interface, you can describe the expected behavior of the interface, and in the class you can describe the behavior of the implementation.
Of course, this is a huge amount of overkill and duplication in many cases, in which case I would suggest documenting in the interface only, primarily because in most cases this is what would be exposed in Intellisense.
Documenting the classes is really only a concern if you will be interacting with both the interface and the classes when developing.
public interface ILoanApplicationRepository
/// Gets the next available application ID.
public int GetNextApplicationId();
public class HttpLoanApplicationRepository : ILoanApplicationRepository
/// Gets the next available application via /REST/Ids/Next
public int GetNextApplicationId()
var req = WebRequest.Create("http://server/REST/IDs/Next");