There are ten key Upanishads and a host of other Upanishads. There is an upanishad that talks about the greatness of the Mahamantra
“Hare Rama Hare Rama Rama Rama Hare Hare
  Hare Krishna Hare Krishna Krishna Krishna Hare Hare”

and this Upanishad is called “Kali Santarana Upanishad”.
This name is significant in that, it bears the name of the Yuga, and hence decries that there is no other go for all of us in this Kali Yuga other than the Mahamantra.

–Sri Sri Swamiji