General boldness:
* As bold as a lion
* As bold as brass
* As bold as a mountain
* As bold as a charging rhino
* As bold as a pirate
* As bold as a knight in shining armor
* As bold as a sunrise
More specific boldness:
* As bold as a declaration of war
* As bold as a new idea
* As bold as a stroke of genius
* As bold as a dancer on a stage
* As bold as a painter's brushstroke
* As bold as a writer's first sentence
The best simile will depend on the specific context you're using it in. Consider the overall tone and message you want to convey.