Formal:
* Awe: A feeling of solemn wonder.
* Wonder: A feeling of amazement and admiration.
* Amazement: A feeling of great surprise and wonder.
* Bewilderment: A feeling of confusion and disorientation.
* Disbelief: A feeling of doubt and skepticism.
* Incredulity: A feeling of unwillingness to believe.
Informal:
* Shock: A sudden feeling of surprise and fear.
* Surprise: A feeling of unexpectedness.
* Stunned: A feeling of being dazed or stunned.
* Dumbfounded: A feeling of being speechless with surprise.
* Baffled: A feeling of being confused or perplexed.
* Gobsmacked: A feeling of being completely overwhelmed with surprise.
The best synonym to use will depend on the specific situation and the tone you want to convey.