Stellan Skarsgard?There are lots of name actors who have voiced dragons- Sean Connery- or have the bass to do it- Patrick Warburton- but I'd love them to go with a new or little used voice, perhaps even one with an accent not usually heard in films targeting the English-speaking market.
German? Chinese? Someone from Africa? Any of this plus a host of others could add an air of the exotic to the drake in question.
Yeah, but then I'd keep hearing Mulan's patriarch ghost...Can't believe I didn't think of this before, but... George Takei.
He's got the deep resonant voice, with just the faintest hint of accent to make it seem "exotic" as compared to the voices of the other characters around him.