One major choice about all books from antiquity is which translation to use (unless, of course, you can read them in original, in which case you don't need my advice). Aristotle's Ethics is available in many translations, but arguably that by W. D. Ross is the best. This translation has been recommended by two separate philosophy professors independently. W. D. Ross is an important philosopher in his own right. His book "The Right and the Good" is the definitive source for Ethical Intuitionism, which was the dominant moral theory in British philosophy for much of the nineteenth and early twentieth century and now enjoys considerable revival. Aristotle's Ethics is a direct precursor of Utilitarianism, of which Ross's Intuitionism is a mature form. W. D. Ross has translated not only the Nicomachean Ethics, but many other books by Aristotle. Thus, in his translation, you can rest assured that the translator knows the whole body of Aristotle's work and is an expert in the field of ethics.