Is it better to perform Dhikr with the heart or with the tongue, and what is the meaning of "the best Dhikr is the hidden one"?

In summary, remembrance of Allah with the heart is a good deed for which a person is rewarded. However, what is better than it is remembrance with both the heart and the tongue, because the one who remembers Allah in this way employs two instruments in remembrance, not just one.