I get why Audrey Hepburn would be toward the top of classic era actresses.
I might be a bit weird on this one for more modern ones, I remember Thandie Newton making an impression on me in MI2. Mila Jovovich ended up high on my list I think largely for the Resident Evil movies. I'm trying to think of who else I might have found 'scary' seductive though and I'm drawing a bit of a blank right now. I was watching Oblivion last night and I really appreciated not actually being able to identify Olga Kurylenko for a while (it shows versatility and talent when the recognition isn't immediate - and it's really unlikely you'd be able to watch her in Oblivion and then Centurion and make an immediate connection, affectation's too different) and for a lot of the early 2010's movies I'd have to give her an honorable mention.
What I would add, beautiful and insane projection capacity, was watching Naomi Watts in the 2008 King Kong where so much of the movie was without words and her affectation was an absolute magic trick (absolute don performance). That's something else that really gets me is when someone is able to go so next-level that you feel like you're watching something that transcends the media.
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“Love takes off the masks that we fear we cannot live without and know we cannot live within. I use the word "love" here not merely in the personal sense but as a state of being, or a state of grace - not in the infantile American sense of being made happy but in the tough and universal sense of quest and daring and growth.” - James Baldwin