Hugo Weaving should play Moriarty in the Sherlock Holmes sequel!
Seriously!
He has a decent height (which adds to the "imposing" factor), a powerful voice (when he says something, you really listen to the words), and an air of intelligence that would perfectly fit the character.
Although it is rumored that the part is either going to Daniel Day Lewis or Gary Oldman, and I would actually be very happy with the latter, I still think Weaving is the best choice for Sherlock Holmes' nemesis.
How's your head, David?
: http://www.youtube.com/watch?v=A_-yBJ0YnZ0