Rooney Mara on ER's 15th season
I remember watching her 2-episode stint (including the episode where George Clooney returns) during the final season of ER and remembering how great she was during her scenes. Her acting was very natural and believable. In the few scenes she had with Angela Basset, you believe her motives and you believe her desire to change.
After watching her on ER, I hoped she would go on to be a star. I'm so glad she has. She's been nominated for two Oscar's and hopefully she wins one. If Jennifer Lawrence can constantly win, so should Mara.