First among all things: a good API allows developers to do something enormously useful within their applications which would be prohibitively difficult to do ab

First among all things: a good API allows developers to do something enormously useful within their applications which would be prohibitively difficult to do absent the API. My two favorite APIs are, by a country mile, Twilio and Stripe. They have wonderful documentation, sane design choices, good first-party library support, responsive engineers on standby for questions, curl-able URLs which return human-parseable JSON, yadda yadda, but the most important reason they're my favorite APIs is that they directly enable me to build and sell the things which feed my family.