I have to agree with Cul regarding Charles Dance. He's only been in two scenes, I think, but he brings forth Tywin's dominating presence.
Iain Glenn looks so good as Jorah. He has not had to display his chops yet, but it seems there is a lot there just under the surface.
Dinklage is doing very well. He really shines when displaying Tyrion's nonchalance at others insults.
This is easy to turn into one of those "Who is your favorite?" threads. I think it is supposed to be about which actors are really standing out from the crowd. And so it's easy to omit the villains in this thread.
I must say that Harry Lloyd really makes me loathe Viserys. He only had one scene this season where he was the main character, yet he really comes through as vindictive, weak willed, petty, arrogant, and mean.
I agree with Devil's Advocate that Jack Gleeson is great as Joffrey. I want to spank that brat. I wonder if the writers will give him the line, "One of these days... Pow! Right in the kisser!"
And even though Catelyn is not a villain, Michelle Fairly's acting really helps bring out Eddard, Tyrion, Jon, and Robb.
And last but not least, I see I'm the only one who voted for John Bradley as Sam. I don't know if the directors and writers are sucking the little life out of Jon Snow or if it is Kit Harrington, but John Bradley is injecting Jon with purpose, honor, laughter, and vitality. I think Bradley is absolutely spot on.