I'm surprised Ellen Page already has three mentions! <3
She's definitely my favourite actress for her work in Hard Candy as Hayley Stark in particular. She's also outstanding as Tracey in The Tracey Fragments, not to mention Kitty in X-Men 3, Juno in Juno and Bliss in Whip It. I've yet to see Inception but I'll definitely watch it if only for her performance.
I don't have just one favorite. :/
I have more then one....so yeah.
So far they are:
Beatrice Miller (fff so what if she's new? I like her voice-acting)
Can't think of anymore on the top of my head, if I do.....then maybe I'll edit this later.