I am going to go with Alex Kingston.My knowledge of British television an cinema is probably not broad enough to make a really good choice. I am probably overlooking some fine actresses who have been in roles outside of Doctor Who I would enjoy immensely if I was aware of them.
Alas, I have to choose from actresses who have made the across the pond jump. Alex Kingston portrayal of Dr. Elizabeth Corday on ER is one of the best acting roles of the long running series, so she gets the nod.
