Emily Riehl's book, absolutely. It's a pleasure to read. Mac Lane didn't have the benefit of decades of people trying to explain category theory. He was writing for experienced mathematicians, and the book was based on lecture series he gave around the world for mathematicians. Also, we know more now and this gives a better context to write category theory for today, rather than what people knew and thought in the 60s. CT was literally just forming as an independent field of study at the time, it was very raw.