World’s best API documentation – what do you think

I’m curious to see how things have changed since I last asked this question, back in March 2012: What’s your favourite API documentation, and why?

Part of the reason for asking is that I’ll soon present a session at STC Summit 2014 on API technical writing. I want to give examples of excellent documentation. I have some favourite documentation sets myself, but it’s great to get the opinions of developers and other technical writers too.

So, have at it! Please comment on this post.

What’s your favourite API documentation, and why?

I’ll also collect any suggestions that people send via Twitter, Google+, and other channels, and add the links to this post.

OT: This has to be the world’s best bird. Well, it’s my current favourite anyway. This is a Tawny Frogmouth, spotted on an early morning walk. Tawny Frogmouths are night birds, about the size of a large owl (34-52 centimetres). During the day, they pretend to be old tree branches.