General disbelief:
* Disbelieving
* Skeptical
* Doubting
* Unconvinced
* Unbelieving
* Astonished
* Amazed
* Bewildered
* Surprised
Expressing a stronger sense of disbelief:
* Dismissive
* Scornful
* Cynical
* Suspicious
* Incredulous
* Unpersuaded
Expressing shock or amazement:
* Astounded
* Stunned
* Dumbfounded
* Flabergasted
* Aghast
More specific synonyms:
* Discrediting: This implies a belief that something is false or impossible.
* Nonplussed: This conveys being puzzled and unsure how to react.
* Bewildered: This suggests confusion and inability to understand.
The best synonym for "incredulous" will depend on the context of your writing.