I've been buzzing about, skimming through these actor discussions. I have seen far fewer films than you two gentlemen; nevertheless, I shall offer my opinions.
As far as the number of quality films a person acts in, I suppose we have to think about the length of a person's career, which I'm sure you've considered. Someone like Elizabeth Taylor, who was quite captivating in her early career hasn't continued. What do we do with someone who has a career like hers? And are you all thinking specifically about leading roles, or can someone be a supporting actor/character actor? What if the person has the potential to be both and has played integral roles and less-visible parts? I guess I'm asking if a person's versatility should be a factor in determining said actor's greatness.
For me, it seems to be about being able to dissolve one's public persona (say, who we the public think DeNiro or Eastwood or Streep *is*) and becoming this other person. That sense of versatility then becomes the trump card when considering greatness, and I think it's particularly remarkable when an actor's appearance (ie his or her physical beauty or age or unattractiveness or something like this) fades. Someone like Joan Crawford I enjoy a great deal, but her later pictures, she's just Joan Crawford being another version of herself (the public persona we attach to her), the melodramatic bitch-goddess. That's why I can get behind someone like Gene Hackman being considered as a masterful living actor.
That being said, I can think of a couple of living actors who may not be the tippy-top of the poll but certainly deserve a second look:
1. Sissy Spacek
2. Tommy Lee Jones
3. Shirley McClaine
Maybe I'd throw in a Sally Field or a Ralph Finnes, because of their abilities to sort of dissolve into their roles.
Also, Judi Dench has a long TV career too. She's quite good in As Time Goes By. And for the record, I'd like to say Paul Newman and Peter O'Toole are two of my favorites. Kudos to considering Cary Grant. I think he was an equally adept comedic and dramatic actor.
Lastly, and this is more tangential, I have less a problem with using the term "actress" to mean "female actor," simply because it's more descriptive and specific than "actor."