What is the best documentation in code you have seen?
This is interesting - when I read your post, I thought, "What a strange question. The best documentation is just the one that explains the problem in a simple, easy to understand manner." You see, I was thinking "in code" as in "// This comment explains the logic in the next code block". And then when I read the comments from others, at the time of this response everyone has chosen that the word "documentation" refers to a "body of knowledge that ships with the product". It's funny how we all interpret this differently.
Not exactly what you're asking for, but Ratchet's landing page documentation is very slick.
Qt has amazing API documentation.
Hands down: JDK.
http://paste.lisp.org/display/133628