What’s your favorite API? Lets get a list going.
For programming or for consuming?
The Remember The Milk API is one I use a lot in my own automations, but I’m sure I use the Feedly Cloud API and the Twitter API much more, though via Apps created by others (Unread, Reeder, and Tweetbot).
Airtable has a pretty good one. It dynamically creates itself as you use bases. So the documentation provides all your fields and data types. It is pretty good and easy to use.