Powerful Mantras are to be found in the Mahanarayana Upanishad.
The Gayatri Mantra,one of the most powerful mantras,which is the Mother of All Mantras
Is found in this Upanishad.
Curiously it is an Upanishad that deals with not only with the enquiry into Reality but to daily Routine and other Mantras for specific occasions.
The Mahanarayana Upanishad is in Krishna Yajur Veda.