We see that question in the Adafruit forums all the time, especially from people who are just getting started. Adafruit has more than fifty boards in the Feather line alone, to say nothing of other platforms like the Metro, Circuit Playground, Arduino, and Teensy. Trying to choose one, especially if you haven't already worked with all of them, can be bewildering.
Our answer, predictably, is: "it depends what you are looking to do."
There's no such thing as a 'best' microcontroller... if there was, all the manufacturers would make that, and there wouldn't be any choice left.
Instead, you have to make tradeoffs between competing interests: The fastest chips tend to use more power than the slowest ones. The boards with the most IO pins are bigger (and usually cost more) than ones with fewer features.
This guide will explain which microcontroller boards are easiest to work with, which ones are more complex, and how to choose between them.