I don't know that I could pick a single favorite actor or actress, really, so I guess I'll pull something out of the air and go with....
Amy Adams in Enchanted
I was going to go with the film Junebug because that was where I first saw her and was amazed by her, but I've posted enough dramas on this list. We'll go with a film that is sheer fun.
I can't remember whether it was me or Brian that chose Enchanted for a Netflix pick, but we both ended up loving it. On the surface, it sounds like a cheesefest: Adams plays a Disney-esque princess who is transported by an evil witch from the comforts of her animated world, where her only concern is finding her prince, to modern day NYC. But, it really works, and I think it does mainly because of Amy Adams. She is totally beguiling. She commits to this character with as much focus as she has any of her dramatic roles. It's impossible not to love her in this film.
It also features Patrick Dempsey (who I totally loved in Can't Buy Me Love back in the day, but I guess I'm supposed to think he's McDreamy now) and Idina Menzel (non-singing role -- booo) as the NYC people she crosses paths with. James Marsden is a total hoot as the animated prince who comes through to NYC on a quest to rescue his princess. Amy Adams totally steals the film though. She is brilliant.
("That's how you know...")