Languages having only one Myhill–Nerode equivalence class
Assuming that you mean the Myhill-Nerode relation, the Myhill-Nerode theorem states that the number of equivalence classes exactly equals the number of states in the minimal DFA accepting the language (if it's regular; otherwise there are infinitely many equivalence classes). So we have reduced the question to the following one:


  Which regular languages can be accepted using a DFA having only one state? 


I'm sure you can answer this one yourself.