Known as the Mahamrityunjaya Mantra, this is a verse from the Rig Veda and is considered to be the most powerful Shiva Mantra. It bestows longevity, wards off calamities and prevents untimely death. It also removes fears and heals holistically.
More on the meaning and power of this Mantra: https://www.youtube.com/watch?v=_htxjk9mlfQ
Om Try-Ambakam Yajaamahe
Mrtyor-Mukssiiya Maa-[A]mrtaat ||
1: Om, We Worship the Tryambaka (the Three-Eyed One),
2: Who is Fragrant (as the Spiritual Essence), Increasing the Nourishment (of our Spiritual Core);
3: From these many Bondages (of Samsara) similar to Cucumbers (tied to their Creepers),
4: May I be Liberted from Death (Attachment to Perishable Things), So that I am not separated from the perception of Immortality (Immortal Essence pervading everywhere).