5. Professor Snape - The Harry Potter Series
JK Rowling's writing of Professor Severus Snape could well go down as one of the most charitably written roles in the history of adapted novels. It was basically tailor-made for Rickman (in contrast to the other lead roles that required casting away from the source a little), and nobody else could have played him. In that regard it would have been easy for Rickman to simply don the wig and black robes and sleepwalk through his scenes, knowing that he had the most helpful safety net possible. He didn't of course, and his mix of malevolence (he genuinely hates Harry, despite his convictions to protect him as Lily's son), superiority and apparent disdain were artfully balanced with heroism, conflict and - ultimately - the revelation of old, unhealable wounds. That's a lot to put into a role that necessarily needed to be shielded until the later films, but Rickman was well-equipped to deal with those unique requirements.