Smaug's voice (done by Richard Boone) and Gollum's voice were done well. Incidentally there's a list of who did the voices in The Hobbit and some of their other work here:
It can be kinda funny if you're watching The Hobbit and you think of some of the other voices the actors have done. For instance if you watch the last Planet of the Apes movie (Battle for the Planet of the Apes) the voice of Gandalf is the narrator :)