Generally, I like to see a high-level overview of the API, authentication scheme, endpoints, sample request and response, error code explanations, and sample code coverage in multiple languages. I wrote a blog post on this topic:
Hope you'll share what you think when we release it.
curl examples are the thing that impress me the most ...
everything a dev might ever want to know about constructing a query, calling this API, and parsing the results--beautifully accessible from this homepage
I also love the Google Maps API. They're certainly in my top 5 for API documentation.