Alan Rickman is a great actor. He burst onto the scene as one of the all time great villains, Hans Gruber, in Die Hard. He would be well remembered just for that, but his career since then has been steady and successful.
Nowadays his role as Severus Snape in the Harry Potter series is probably his best known. But he also made a wonderfully evil Sheriff of Nottingham in Robin Hood and was fantastic in Galaxy Quest.
To me, though, he'll always be Hans Gruber. Great Actor!