I'm one of those people who would put Williams over Dinklage.
Not to knock Dink----he's everything you wrote about and more---but Arya Stark's character is such a demanding one and Williams pulls it off with the effortless charisma of a young Kate Winslet(Go watch Titanic again, Winslet plays the haughty, over privileged under-valued woman without even trying).
Arya has to be a strong tomboy, a bratty little girl, an actual boy, vulnerable...when Littlefinger shows up she has to be engaged while being as absentee as possible...all these things with half the scenes of every other "major character".
Those scenes with Tywin are incredible because of all the things Williams has to do: she has to show her respect for a man she could see herself looking up to while barely withholding her contempt and hatred for him and his family. She's trying to be childlike and oblivious but episode after episode more of the facade leaks out and eventually her and Tywin can barely pass their intentions off as subtlety.
She does so much with so little that it's quite hard to fathom.
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Realo won gold at the Sex Olympics with a BROKEN FRIGGIN NECK.-Voltch