Being bigendered and all, it's hard to choose a voice that isn't male nor female.
Though if male, it would have to be Roger Jackson:
As the Cheshire Cat (http://www.youtube.com/watch?v=wBO5aO6iJlk#ws)
And if female, maybe Cate Blanchett:
As Lady Galandriel (http://www.youtube.com/watch?v=Y47IBN7UGYo#)