Of course, his eight film role as Severus Snape is probably the best remembered. His presence (and others, especially Maggie Smith's) certainly lent that fantastic tale a weight which made such a magical world feel real. Granted, the part was literally written for him; JK Rowling not only had him as her top request for the part, she even wrote the books with him in mind as she created Severus's character.
I certainly understand Rowling's choice, as Rickman was who I had in mind when I created King Aristain in my own magical screenplay: The Rogue and the Bride. Granted, I never expected that he'd play the part, but it's his voice I heard when I wrote the lines (and still hear as I read them).
It's strange that, although I associate him most often with villains, I am so deeply saddened by his passing.