This is not going to an easy task, as defining what is best is largely subjective. More than that, there are so many great works of literature out there. With this said, Pygmalion has two important things going for it.
First, the story is old. In other words, it has passed the test of time. To the best of our knowledge, the first Pygmalion appeared as a myth by the great Roman poet, Ovid. He lived from 44 BC. to 17 AD. It was a hit in his time and it continues to be until our time. So, this fact alone makes it great. The story touches people from many different generations.
Second, there is something about Pygmalion, the play, that strikes a chord in our hearts. In other words, there is something eternal about the work. Just as professor Higgins seeks to transform Eliza Dootlittle, we think we can master the world as well. More importantly, we actually seek to do it. This will always be the case for humanity. For this reason, this play will always strike a chord in our hearts.