Formal:
* Equanimity: A state of mental calmness, composure, and evenness of temper, especially in a difficult situation.
* Serenity: The state of being calm, peaceful, and untroubled.
* Tranquility: A state of peace and quiet.
* Composure: A state of calmness and self-control.
Less Formal:
* Placid: Calm and peaceful.
* Unruffled: Not disturbed or upset.
* Sedate: Calm, quiet, and dignified.
* Serene: Peaceful and tranquil.
Figurative:
* Zen: A state of profound peace and enlightenment.
* Nirvana: A state of perfect peace and happiness.
The best word to use will depend on the specific context and the tone you want to convey.