In fact your question is a very tough one and very confusing!!
However, I guess Vivien Leigh is the BEST answer to your question.
I chose her on account of her personal achievements including 2 Academy Awards for Best Actress in a Leading Role in 2 spectacular roles in the movies :
1. Gone With The Wind. (1939)
2. A Street Car Named Desire. (1951)
Other outstanding actresses may include Meryl Streep, having most Oscar nominations more than any other actor (or actress) in the history of cinema industry ...
Another one could be the gorgeous Sophia Loren ( 1 Oscar) or Elizabeth Taylor ( 2 Oscars).
Yet I still think Vivien Leigh is the best on account of her outstanding roles, but this is just me =)
I think I'll make a hub about the best actors and actresses very soon ... So check it out!