While I yield to no one in my admiration for Ms. Staunton, I respectfully disagree that she was the only one who nailed the character she played. What about Alan Rickman? His portrayal of Snape impressed JKR so thoroughly that she shared secrets with him about his character that hadn't yet even been published!
shareRickman's Snape was too underplayed. He was really nasty in the books. In the films he was barely condescending.
shareI see your point, but I think that's mainly a function of the screenplay vs the novel. They can only work with the script they are given. On that score, Rickman was aces.
There was so much great acting in this series. Others who nailed their parts:
Richard Harris as Dumbledore
Maggie Smith as McGonagall
Ralph Fiennes as Voldemort
Robbie Coltrane as Hagrid
Gary Oldman as Sirius
David Thewlis as Lupin
Jason Isaacs as Lucius Malfoy
I'm sure I'm forgetting some more.