Erm. Most of the actresses who I really really love (e.g. Holly Aird, Jane Horrocks, Jodie Foster, Sophie Aldred) are people whose careers I've followed quite regularly for a while and are accordingly about my age or older (or, of course, in one case, dead. :-( See userpic).
As far as younger actresses are concerned, I've had a think, and the one person who really springs to mind is Keeley Hawes. I've seen her in a few things, and she seems really good, and also (sorry if this sounds shallow, but) she has a really pretty smile.
Other contenders would be Karen Gillan, Freema Agyeman and Anjli Mohindra, all pretty much exclusively for their Doctor Who-related roles; Scarlett Johansson I quite like, but as the only things I've seen her in have been The Girl with the Pearl Earring and Lost in Translation quite a few years ago, and The Avengers more recently, and it's the latter which really made an impact, I'm not sure that's enough to form a decisive opinion on. Also, I'm quite interested to see Kirsten Stewart in Snow White and the Huntsman, but as the only other thing I've seen her has been Panic Room (in which she was, admittedly, darned good), I don't think I'm well-placed to form an opinion. Also, Chloe Moretz made a really kick-ass Hit-Girl, if you'll pardon the pun. Again, though, that's all I've seen her in.