One of the most common compliments directed at Meryl Streep is the hyperbolic phrase
the greatest living actress/actor, Google reports 24,800 instances in connection with Meryl Streep. The term actor is perceived to be more neutral and respectful and is no longer confined to men. In today's politically correct society, the term "actor" is considered gender-neutral.
Oxford English Dictionary (3 ed.). November 2010. Although actor refers to a person who acts regardless of gender, where this term "is increasingly preferred", actress remains in general use; actor is increasingly preferred for performers of both sexes as a gender-neutral term.
And in the article dedicated to the actress, it gives the following accolade
she is widely regarded as one of the greatest film actors of all time
A one word expression that is often heard in these circumstances is superstar, with its connotations of greatness and immense fame. Megastar is another alternative.