As for services, I think Parse is a good example of a very well written documentation.
http://www.sqlalchemy.org/
http://www.makotemplates.org/
http://docs.pylonsproject.org/en/latest/index.htmlThey also had usage examples so that it was easy to remember "Was it find(needle, haystack) or find(haystack, needle)?".
jQuery is the same way, though their comments seem less curated, and often have red herrings attached to the documentation that should be removed, but aren't.
Really good documentation exists for Django and Python, but in my opinion, more 'how-to' examples would be great for people just starting out.
That said, since you asked what the best is, for me, the answer is quite easily the documentation for Stripe[1]. In addition to being exemplary in every other way, they also do real-time substitutions for variables.
An example that would look like "get("<YOUR API KEY">") in every other document you've ever seen would actually contain YOUR API key in the Stripe.com docs. Simply brilliant.
Leftronic[2] does the same thing as well, which I thought was a good lesson learned, but overall, their docs aren't quite as brilliant as Stripe's.
[1] - http://stripe.com/docs
[2] - http://leftronic.com/