I'm actually very interested in what other people's favorite casting choices are. Personally, I'd say Tywin was one of the most irreplaceable, but Tyrion, Littlefinger, and Catelyn (for some reason, I think her performance as Catelyn was spectacular). All of the actors except maybe Shae were great in the roles, and these aren't necessarily my favorite portrayals of GoT characters, just the ones I think were irreplaceable.